The mixamp isn't really overpriced, you just already have something that does the same thing. It's about the cheapest way to go if you want positional audio over headphones on a console. You don't need that though.
As for other amps, how much do you have to spend? I don't know if there are any better amps for much cheaper than the mixamp, which you thought was too expensive. AFIK fiio has the ultra-low-price market pretty much wrapped up. The only amp in the sub $100 price range you could say I have any experience with is the Headroom Airhead. I don't actually own it but I do have the Bithead, which is the same thing plus a usb DAC. You wouldn't need the usb part unless you also wanted to use it for music listening on your laptop or something. It will power anything from low impedance IEMs to 600 ohm monsters.
If not that, then a CMoy. Depending on who you buy it from and the quality of the parts they use, it may be quite a bit better than the Airhead. I don't know who's selling the best ones these days, or exactly what they go for. I'd bet you could get a quality one for cheaper than the Airhead, but you'd have to ask about it in the portable amp forum, since everything I mentioned besides the mixamp is pretty small. There's also probably some other things I'm not mentioning because I don't know about it, so don't this as authoritative.