It's all relative.

With a lower budget, your return expectations are much lower. With a large budget, your expectations are extremely high. The larger hosts have to have their ads perform each and every month to obtain the needed quotas for purchases, growth, revenue, etc. on a much higher level. (I would lean to the side of it being a bit harder for smaller companies as larger companies tend to get the chance of having their brand already established, and much larger WOM possibilities, thus driving in more results for less advertising).

Start small by dabbing into several things, work with the sites you wish to advertise with on test campaigns and see how they work out. The key is tracking, always track... visitors, exposure, conversion %s and of course... most importantly, CPS.

Then weed through the useless slots and increase on your bigger ones.