Or maybe ten bucks a tag, divided by the character's resources.  Seems like a perfectly fair way of doing it.
First, determine what your expected number of 
Wads of Cash will be.
Next, estimate the amount of money you'd need to make the payments on a new BMW.
Divide the second number by the first to get your cost per tag!  Suggestion: double the result, just in case.  Blame it on the economy.
If actual useage lags, options include:
1) Allow flexibility in what 
Wads of Cash invokes can be used for.  "I cast 
Magic Missile.  I attack the Darkness!  Here's three 
Wads of Cash for a +6 to my roll!"
2) Allow 
Wads of Cash to be used to buy off compels.  COMPEL OFTEN!  "Hm.  I see that you are a 
Wizard of the White Council.  Well, unfortunately you just accidentally hexed the high power cables, and one snapped and is heading 
right for your head.  (Holds up a chip.)  Ah, buying it off with 
Wads of Cash, eh?  Then the wire narrowly misses you.  But then, there's another CRACK! as the next cable over snaps and is whipping right toward your head!  (Holds up a chip.)
3) Normal invokes now cost a Fate point PLUS a 
Wad of Cash!