What is the average FPS for Metro 2033?

Just go into "settings" and set the first person shooter to 60 units.

Then you should be good to go.

It depends, but 60 fps is always the best for you. Almost all LCD displays use this refresh rate, so it's useless if play on more fps than that. Although Metro 2033 is pretty though game for gpu, so you might want even lower fps to get the better graphic. It depends on you what's is smooth enough. I'd say 50-60 fps (stable of course), it works best for me. Although I know many people that play on 30-40 fps and they're ok with it. I wouldn't recommend to go under the 30 fps for sure.

Hmm my 200 euro(I do buy my parts separately though) quite old PC can run it on a steady 40 fps on maxed settings. Which is decent for a single-player game, since it's smooth enough for its slow pace. Always this nonsense "800 dollar pc needed" crap. People really have no idea..

Too bad I lack $0.79 for humble bundle, so I can't even try it

This is basically the samething as questioning: "What's a brain?"

There's a huge difference between DirectX 9 and 11 in Metro 2033. If you set DirectX to version 9, even with an average PC and very high settings, you'll get more than 30 FPS, which is completely playable.
If you set DirectX to 11, IMHO you will get slightly better graphics quality, but you'll need a good PC to get average fps of more than 30.
My FPS was about 45-60 most of the times with AMD Phenom II X4 840 and NVIDIA GTX 260 and DirectX 9.

By the way, Metro 2033 is one of the best games I've played in recent years. :)