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.