As mentioned, there are cheaper supplies to be had if you have some tools and are willing to spend a little effort.
The Proofgrade does have a more reliable consistency though (no hallows, knots, facing plugs, etc), plus you do not have to waste time surface sanding for the masking and sizing the wood for the bed.
I do 4x8 sheets at Lowes or Home Depot, and overall they can be under $2.00 a board. The store will rip it into four 20 inch sections for you with a 16 inch short section (even if you got a table saw at home, this makes them very easy to lug out of the store).
Once you get home, convert those 20 inch slaps into 4 12x20 sheets and you have an impressive pile of 20 boards when done.
Now the fun. Clean up the edges and lightly sand the faces of the good looking boards (for better masking attachment). Both sides if you think it will be something important. Make a separate pile of any boards with defects, knots, etc, for scrapper trials since they will not need masking.
So there you go, how much is your time worth? If willing to spend it as mentioned above, you can get 20 boards for 1 to 2 bucks apiece, depending on which ply you buy.
Even an expensive sheet at $30.00 plus a$10.00 roll of blue painters tape (if using that) still only costs 2 bucks each if you have an afternoon to spend on it.
Woot !?
OR --> Just go to the Glowforge, pop in a chunk of Proofgrade, and vapor that puppy.