The best way is get on there and try out a few keywords that you think will work. You most likely have a few in mind anyway. You could always try the
Google adwords keyword tool
I would probably pay a little more per click to start getting some CTR and then reduce my CPC down once you have a few clicks going through and you have optimised the keywords, ads, and relevancy. Doing this will reduce you average cpc.
A few things you can do to control/improve your spend.
1. Set a monthly budget
2. Set individual costs per keyword
3. Improve ads, keywords & relevancy by increasing CTR
4. Add some tracking code to your site so you can see ROI
5. Add keywords to your ads to improve CTR
6. Capitalise ad text to improve CTR
You can add the searched keywords to your title or add text by using
{KeyWord

efault Keyword} as the title or part or your add text. If the keyword the user searched for is shorter than what's available then it will show in the add otherwise you will get the default. This helps improves CTR about 25%
by the way... the smiley face is a colon but i couldn't work out how to put a colon there without the smiley