What am I getting out of a $55 chainring vs a $25 one? Are more-expensive ones longer-lasting, or more lightweight, ...?
This is for a $700 single-speed, to give you an idea of quality of bike I'm currently using (e.g. Not some $3000 professional racer)
You get what you pay for. Cost is going to be dependent on materials, engineering, etc. If you buy a cheap chain ring it may work pretty well if you don't demand a lot out of it. A more expensive chain ring will be made of better materials (will wear longer and be lighter) and be better engineered (usually an issue for shifting; not so much for a fixie).
When it comes to parts or tools I am usually dead set against buying "cheap" since you're not just paying for the materials and engineering but also the service and reputation. If you can get a decent part from a reputable place by all means go for it, but don't buy the cheapest part you can from some shady on-line guy and expect to be able to get any help when the mounting holes don't line up.
If price is your prime consideration you may want to pay a visit to your local bike shop. Spare chainrings are not a hot seller and there may be a chance that your local shop can do you a deal on a ring that has not sold or had any interest from customers.
On a single speed I don't think top quality is your prime consideration from the chainring. You have two material choices - steel or aluminium. The steel ones come in cheap and expensive 'ultra durable' flavours. We all know that steel (whether stainless or not) is heavier than aluminium (not that it amounts to a lot of difference on a chainring), so take a look at the aluminium offerings. Remember that you can flip a worn chainring over in some cases, to get twice the wear out of it.
If you are going the local bike shop route, make sure you know what BCD means. You want the holes to match up to your crank spider, and for there to be the correct quantity of holes. You also want to make sure there is no wow and flutter. Surface scratches can be easily removed (except on black anodized rings) so have a root around in your LBS and ask what they have knocking around in the workshop if getting a deal is of your prime concern.
Hope that helps.
I generally like to buy the OEM parts, unless I'm consciously "moving up" to something of better quality. Something like a chainring is generally "tuned" to the style of derailer, etc, originally supplied on the bike, and changing the type of ring (without due consideration) could lead to poorer shifting or other problems.
(I say this even though I don't especially like the monopoly control that Shimano has over the bike industry, and the above rule makes me tend to go back and buy more Shimano parts. I just kind of grit my teeth and do it.)
