Hi,
I hope I understood, what you trying to do.
Assumed you have the fields (RoadBlocks, a, b, rb and UserInput) then the script would look like:
if (RoadBlocks eq 1 and a eq 12) then
$ = rb * 7500
elseif (RoadBlocks eq 1 and Within(a, 13, 21) eq 1) then
$ = b
else
$ = UserInput
endif