Try some cheap words first...maybe they aren't even related THAT much. Make sure you put a conversion page in so you know how many leads turned into sales.
If you get a decent conversion rate up your bids or try more expensive words.
If the conversion rate is low try more targeted words for a SHORT period to test the convertion rate.
It is VERY easy to blow through $50 in a couple days with 0% conversion if your not careful.