air mileage questions

I'm considering applying for a credit card providing air credits, such
as Chase explorer, MC Saphire, or another that provides credit for
United/Continental flights. I find lots of ads, and several comparison
tables, etc., but nothing that explains clearly how many air miles I
would get per $$ of purchases, or initially. I have tried to find an
explanation of what "miles" means with any of these cards, but it's a
complete mystery. In other words, for 40,000 bonus "miles," how much
credit do I get for a United ticket? Or, assuming it's $.02 per mile as
someone suggested, does that apply equally to all United flights (and
all included airlines). Does it apply in all seasons, to all flights,
and does it apply to flight prices already discounted, including foreign
flights? (For example, I'm interested in flights between Houston and Paris.)

- - Granted it's a basic question, and maybe I'm the only one who
doesn't know the answer, but it's rather significant if I want to
compare the benefits and costs of different cards, and I don't see it
even mentioned in any of the ads or articles.

Thanks for any suggestions,

Jim