Well first of all Microchip provide free samples in small quantities.
I've received everything from pic32s to dsPic to 12C508s for free.

You can only really tell how much RAM you need with trial and error where PBP is concerned.
It's own asm routines added to your program to make the BASIC command lines work are
never really seen without disassembly of the compiled program.

That being said, it isn't too difficult to tell how much RAM you have LEFT on the chip you are using.
Just define a bunch of useless variables before a critical variable is defined
(one that the program will fail without).

An array is good for this, just up the number of elements until no more can be assigned,
then you know bank 0 is full, do it again to consume the rest of the space until the program fails.