I would apply a formula along the following lines:
1) Work out your rpv (revenue per visitor) - or at least how much you are prepared to pay per visitor. If you are bidding on espotting, Overture or Google Adwords, you have probably already done this exercise. My sites which are affiliate based generally don't earn enough (yet) to justify any of these, so let's say your site makes 4p per visitor...
2) Use a site like
www.wordtracker.com, or a programme such as "Good keywords" to work out roughly how many people are searching on relevant terms to your site. Let's say there's 10,000 relevant UK searches on your topic.
3) Previously, I'd go on 20% of those searching on Yahoo, but that figure is much lower if you take out the Espotting and Google fed results. Let's say 5%, so 500 relevant searches.
4) Find out how many other competitors there are, and if you can get a description which makes your site stand out (often difficult)
5) If you assume your site will do "as well" as everyone else, let's say - there are 10 sites, you may get 10% of traffic (more on this later).
6) You may therefore expect 50 visitors per day.
7) These will be worth 50 x 4p = £2 per day

You are spending £200 for this, therefore:
9) 100 days to pay back your investment.
Make your own decision from there, but at least make sure your site conforms to the Y! guidelines, otherwise you will, of course, be guaranteed to chuck your £200 down the drain!