I think he ment a cross, they are both one of the six subspecies of the same snake, Morelia spilota ssp., and sometimes breed with eachother in the wild anyways. Diamond, Jungle, and Coastal are the most commonly kept and bred ones in the U.S. I would suggest you give them a bigger tank though. If im thinking right, carpet pythons are a little arboreal, but dont quote me, lol. Either way, that tank is way too small for a 7ft python. Plus they are kinda shy when young, so having the tank open on both of the long sides could be very stressful.
Now, as for the price, check out places like reptileauction.com (scroll up to the top of this page and click the blue rectangle on the left side that says "auction".
Heres a pair of younger ones for sale, current bid is $100.
http://www.reptileauction.com/cgi-bi...tem=1134937895
Heres a dutch auction for $300 per python:
http://www.reptileauction.com/cgi-bi...tem=1135270310
Heres a related species* for $175:
http://www.reptileauction.com/cgi-bi...tem=1135871131
*Morelia bredli, grows to 5-6ft, very secretive, found in trees and on rocky outcrops near water.
Also you could go to a reptile expo and see if there are any breeders in your area. That would be the cheapest way, plus you get to talk to the person who hatched the snake AND you get to pic your favorite out of a group.
PS- I dont know very much about carpets, so check some books or surf the web for more info