If you haven't got a lot of money to spend, a cheap mic is better than no mic IMO!
There are 2 things an expensive mic will get you a cheap one won't - less hiss and better response (no bias towards low frequencies). I've never used small diaphragm condensers but I've heard quite a few recordings made with them and they do sound pretty good, especially if you get a matched pair and record in stereo.
For the recording I do, I use a large diaphragm condenser. It's a samson Co3 that cost me 30 quid. I suppose at this point I should put my hands up and say it's not a proper condenser in the Neumann sense, but it sounds a lot better than a fully dynamic mic. You'll find you need to spend a bit more time getting the mic positioning just right. I find about 6 inches from where the finger board meets the rosette of the guitar is a pretty good compromise between volume and avoiding the boomyness you get when you place the mic directly in front of the soundhole..
The best condensers you can get for very little money are made by Rode. The Rode NT1 is a proper condenser for about 90 pounds. It certainly beats shelling out 2 grand on a Neumann.