I am about to finish designing logo. I use two fonts. One I already bought, but the other... I need just one letter from this font but it is not free.
Do I need to buy the entire font?
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If you have the font on your machine to make the letter but didn't pay for it, then installing the font was your infringement. But there is no copyright protection on the shape of letters:
Under U.S. law, typefaces and the letter forms or glyphs they comprise are considered to be utilitarian objects whose utility outweighs any merit that may exist in protecting their creative elements. Typefaces are exempt from copyright protection in the United States (Code of Federal Regulations, Ch 37, Sec. 202.1(e); Eltra Corp. vs. Ringer).
You can read the article for the full details. Font files are considered to be effectively computer programs, and protected by copyright. But if you convert the glyph to vector--and avoid embedding of the OpenType/TrueType font file in any file format you distribute--you would almost certainly be fine.
Perhaps a bit paranoid: but if you consider cases like Adobe vs. Southern Systems (via @DA01), it may be possible the vector fundamentals of the drawing program you were using are directly mapping font control points in Convert to Vector. But a single letter in a drawing that can be forensically shown to consist of the same "control point DNA" is a far cry from the details of that case, where they created thousands of commercially competitive derived font files from Adobe fonts! Still if you were worried about this nuance, you could convert the letter to bitmap at a large size, then convert to vector.
A greater issue to consider may be whether going in with your direct selection and path editing tools could let you tweak the design of this letter to be better. Is it missing alignment points on your grid by a hair here or there? Is the thickness exactly right, or is it off and you're afraid to edit it? For typographic marks it's okay to start with letters that are "close" but at some point you want to convert to vector and edit it for what best serves the design.
Most typefaces are sold as an entire set, so yes, you'd need to purchase the entire typeface.
In the grand scheme of things, a typeface shouldn't be a make it or break it part of the budget. It's just yet another tool that you'd be using to produce the final product.
All that said, there are alternatives. For instance, House Industries' PhotoLettering service is designed to let you buy 'per letter' as needed: http://www.photolettering.com/
WARNING: SOAPBOXING AHEAD!
It's best to treat typefaces as really important tools. It's like a carpenter that invests in a quality Japanese saw vs. the $12 one from Home Depot. Both can 'cut' but one is going to make things so much easier for the craftsperson and the investment in the Japanese saw is going to pay off dividends down the road in terms of better quality output--not to mention saved time and sweat.
So when debating whether or not one can afford a typeface, one should really ask themselves if they can afford NOT to pay for the typeface.