I'm in the "300% markup" group when it comes to native files.. here's why...
The cost of the original document design. This is a minimum amount of time you will save someone else by providing the file(s). If the piece had to be created from scratch... this is what it would cost. Providing native files, for me, is not an alternative to the client paying for recreating a file.
The ease at which the original document can be edited/updated for years to come. Every time they want to make a small adjustment, they have the files so it's a quick matter to edit. Native files offer this ease in ability. It's a commodity and has value.
+ 100% ------
Ease of repurposing -- As they put it to you "they want their employee to use them for other promotional pieces for their company" They are essentially capitalizing on my creative abilities and aesthetics. Providing files where they can easily copy/paste elements from rather than taking the time to create elements means they want to use my design sense but not pay me for that design sense. Again, this is a commodity and has value.
+ 100% ------
Utilizing my design aesthetic without my control. I can understand if some feel this 100% based on "style" may be unwarranted. To be fair, this 100% is somewhat flexible to me. I don't always calculate it. It depends on how much advertising I've done based on the design and how ingrained my own personal promotion materials are with respect to this project. However, if I've spent a fair amount of time pushing my services using phrases like "designed the XXXXXX catalog since 2015" and now.. the 2017 catalog looks horrible because I gave the client the files and they had an intern mess it up... that may be costing me business in an indirect manner. If a prospective client sees my advertising, then goes looking for the catalog..... I have to stop, edit or otherwise rethink any existing advertising and promotional materials.
In addition to the above, I often create unique, original images and illustrations for my projects. Providing these images as part of the overall delivery has extreme value. I created an illustration or illustrations for a specific use in one specific piece, not for everything they may want to use it/them in. I would essentially be giving the client unlimited usage rights to my original artwork as well by providing native files. If a piece has even one original illustration from me, this 100% is factored in. If it's loaded with original illustrations, 100% may not be enough here.
+ $???% ----
Third party items. Legally you can not provide fonts or licensed images. For third party items I tell clients I can provide links where they can purchase the items themselves, or that I will go purchase another license for their use. In some cases, these third party fonts and images can easily be several thousand dollars alone. In addition, if the client provided images you can not charge the client for those images, obviously. However, if you edited or otherwise corrected the images, there's value in returning clean, usable images to them. Images which could then be used across all their promotions because you've had your hand in making them acceptable for use.They want their business to otherwise benefit from your work outside the original agreement you had.
($X times 3) + (Third party items) = native file cost.
Ultimately each piece and client is unique though. These are just the general guidelines I base pricing on. I look at the piece in question and decide what is applicable in this instance. What benefits is the client getting by having native files? How unique are those benefits because of my abilities?
At a minimum I tend to almost always calculate the 1, 2, 3 and 5 for native files. 5 always since it's an automatic additional costs for legal reasons. For a piece that is pretty type driven and not very complex I may waive #3 as well.
I recently created a product "idea book" which was 95% client provided photos and very minimal type. They wanted native files, in this instance I actually charged 1/4 of $X. All client photos and really all they were getting was how I organized the images in the format size, not anything proprietary or unique to me beyond that. I didn't even adjust most of the images beyond converting them to CMYK from RGB (high quality professional photographs were provided).There was some of my aesthetics in the overall layout, but unlike many of my pieces, this one contained no original imagery or unique items I created just for this project.
Nothing's set in stone. I've lost track of the "but we paid for the design, don't we own the files" conversations I've had. Most clients eventually understand - some still work with me, some don't. I am always willing to negotiate with the client openly. The pricing I calculate is more an "asking price" for me. I'm open to reasonable arguments against the pricing. However, once explained logically to clients, their arguments tend to fade away. (If you hired a baker for a wedding cake, you get the cake... you do not get the recipe and a design schematic for the cake.)
I won't sugar-coat this and claim it's always an easy task to get paid for native files. In fact, just the opposite. It's often a challenge but the more you do it the more you learn how to combat the "we get everything don't we" mindset of some clients. I state on my contracts that native files are not part of deliverables and that seems to assist with this a great deal. After the fact, it's a negotiation in almost every instance.
The folly of new designers is providing anything and everything free of charge just because the client asks. Easy to do when you are starting out or working freelance in your off-hours from your 9-5 job. But it becomes very difficult to maintain that mindset once your livelihood depends upon your files. I make a concerted effort to get all new clients to understand that I never provide native files free of charge.
IMPORTANT: I should also add... never, ever, provide files until payment has been received. I don't care how good a client is. You should have a blanket policy that payment must be received prior to files being delivered.