Personally, I would look for promotions and free credit deals... I have seen some free $75 when you deposit $5.
I would do something along these lines and just see how it goes. There is no point in paying thousands if it goes no where - just do the bare minimum and see what sort of increase you get (Be sure to track and have custom landing pages e.t.c.)
If for example, for every $10, you get $100 extra profit per order / order increases - go with it, pay a lot more!
If you are getting no increase in business and just spending money like crazy - don't go ahead... You may have more luck looking for the sort of places where you can just pay 5c per thousand impressions or similar.