I don't think there would be anything WRONG with permitting a lighter shield with the usual DB and cost, but a risk of breakage. Say:
Lightweight*: A wooden shield can have its standard DB at only 1/3 of its usual weight. Such a shield rarely survives past one combat. Roll 1d after any successful active defense anytime the shield adds its DB; it survives only on a roll equal to or less than DB+2 on a dodge or parry, DB+1 on a block. It's also ruined if it burns for seconds equal to reduced weight. Finally, it gives -1 to damage in bashes and shield rushes. For instance, a lightweight medium shield is DB 2, $60, and 5 lbs.; it breaks on roll of 5-6 on 1d on a dodge or parry, or 4-6 on a block, burns to a crisp in five seconds instead of 15 seconds, and bashes for just thrust-1 (no better than a punch). +0 CF.
* Lightweight is mutually exclusive with dwarven, meteoric, and orichalcum. It means the shield is of thin wooden construction.
This may look too good to be true, but in reality it's a huge drain on cash and encumbrance, because the fighter will probably end up needing one shield per battle, which means carrying extras. That might even be
realistic.
And yes, the breakage rules are hugely simplified for the
DFRPG, which doesn't track DR, HP, HT, etc. for shields. It also means that you can lose shields to low-ST halflings with daggers, arrows, and so on. If you want, replace my simplification with one that compares attack damage to DB in some way (maybe "Roll damage normally; if basic damage exceeds 5×DB, or just 4×DB on a block, the shield breaks."). This would need playtesting.