What websites are similar to shutterstock but cheaper for commercial use
Well there is many Freemium websites for vectors which can be used for commercial uses :) below are the list sorted by it's quality as i've experienced.
4.All Free Download
6.1001 Free Download
and not but least 7.PNGTree (quality is good but maybe you will be scammed by getting flattened .psd instead of requested vector but it's PNGs are top notch)
about freepik: is the free assets are free for commercial use? Is the premium membership in freepik are worth paying for?
Free assets are available for commercial use also by attributing the designer although they recommends not to use their resources directly and I'll also suggest to do same :)
By the way if you asking for purchasing premium (only if you can afford) then i'll say : "yes do it but just don't RUSH" since the freepik company usually announces coupons upto 50% on occasions and once you have 50% off voucher just purchase and Enjoy! and not but least you can partake in their competition where they giveaways Premium Plans Even i got my premium by GIVEAWAY.
Is there any other suggested websites for vector/eps assets for mobile games use?
I'm also on same page since I'm also game designer and after my experience all i can say you is "Never find readymade UI"
freepik along with vecteezy and vectorstock have more than enough assets which you can use as secondary elements or can make composition by your creativity and it all will give you a cool User Interface! Happy Designing. Hope you got your answer :)