Well, that's what the MFA in Meta charges me! They will not have the 7.00 by 20, That size will require a specialty supplier if you insist.
They will have the 7.50x20 and the 8.25x20. I use 7.50x20 all around on my 45. Price for those has been $180 apiece, includes the Flap, tube separate. You will find that is a very good price indeed for these tires, mine have been excellent.
You can always start out with 4, get the last two later. Do you have the 2 speed rear end?