Yes, they should be counted in with WBL, and yes 50gp x spell level is too cheap - and it's not the rules. To copy a spell is USUALLY 50 x gp, sayeth the SRD, which means it's whatever the DM wants. Not to mention the fact that that fee is just to copy the spell - materials cost to put it in your own spellbook is 100gp per level.

So 150gp x spell level minimum for those "interpret every rule in the wizard's favor" types. Not a huge amount, but for the "Batman wizard has spell for every occasion" types, that'll be a noticeable bite out your magic item budget.

Of course, one also has to take into account the possibility of failing one's spellcraft check when trying to copy a spell. I doubt most wizards loaning out their books give refunds.