I'm not sure how good my logic is here, but my guess is he's charging $200-$250 to sign bats.

$40 for standard sportscards through Tigers Foundation (until yesterday - as of today, they will return all envelopes to sender; new pricing will be announced in Spring, I think.)

Miggy had a signing on 14 Aug at DC Sports. The pricing is no longer posted on their site, but they do have prices for items that had been signed:

16x20 photos: $324.99
Generic OMLB Baseballs: $274.99
Generic Bats & Black baseballs: $399.99
Bats with TC inscription: $524.99
Game Model bat (unused): $749.99

They have a Rick Porcello signing coming up with the following ticket prices:

Flats (up to 11x14) & balls: $35
Premiums: $45
Inscriptions: $20 per five word phrase

They also have items available for pre-order.

8x10: $46.99
Baseballs: $61.99
Jerseys: $129.99 (#48) & $169.99 (#21)

All of that information led me to believe they were asking $150 for flats/balls. Then add another $25-$50 for premium item and another $25-50 for the game used inscription. I'm guessing that based upon the spread of $110 (give or take) on Matthew Stafford signed helmets in stock (from his last signing) and pre-order (for his next signing).