Proton (PDS) is a PBP-like basic compiler that has GLCD support. For reference, a very short program takes up about 750 program words, about 1/3 of which is the GLCD commands, half is the font file, an the remainder is the basic code that you actually type. Additional GLCD command use takes very little space. Just like all of the other commands, adding the first one is what consumes the most overhead; additional commands just load registers and call the existing subroutine.

In any case, this really is not very much memory, considering how many PIC's have so much more than 4KB of space, and they go on up to 64KB or more. The amount of overhead really is irrelevant, unless you are counting your pennies on the PIC--and that's unlikely if you are using basic to control a GLCD.

The real issue is that PBP is more designed to be a beginners tool, primarily for those stepping up from Stamps. Most of those beginners don't have what it takes to make a GLCD work, with or without the basic commands. If you must have built-in commands, look at PDS or SF for Basic. PDS has the advantage of being as simple as PBP, but more powerful. SF is no where near as simple, but it's more capable.