Does anyone know if there is usually a minimum CPM buy? I.E. --> CPM rate is $10.00, is it standard to have to purchase a minimum number (say like 100,000 which would be 10.00 x 100 = 1,000.

CPM (cost per thousand ads) depends on a lot of factors.

Since every advertising company has different rates, contact a bunch of them and find out.

Well I know the rates will differ and I've investigated some rates, but do they usually make you buy so many thousands?

Every company is different with different terms.

If you are serious about that kind of marketing, you should want to be exposed to as many thousands of eyeballs as you can get for your money.