I'm all for "support your LBS" but $20 for one of those pressed out pieces of tin seems ridiculous. For the record I've bought tools at my LBS and paid prices that although more expensive than online, certainly weren't that ridiculous. For example I remember buying a Park PW-5 pedal wrench for $14, and I think they go for around $11 online.

BTW I don't know how accurate it is, but this website seems to suggest the MSRP on that tool you bought is $4:

Park Tool CNW-2 Chainring Nut Wrench > Accessories > Tools > Crank & Chainring Tools | Jenson USA