61 hours is alot for sure, But these 700-1500 bills can be way low IF......
There is a ton of under coating on the bottom, Undercoating sucks to remove with a blaster and takes FOREVER ( Insert SANDLOT voice ).
If there was tons of Bondo, and you had them blast it to be removed instead of grinding it out or just cutting out the panel that obviously needs to be replaced if there is that much MUD.
Surface rust takes the longest to remove of all materials, Rust means you have to go further down to find fresh metal.
If they used a media like soda that is soft and will not hurt chrome and stuff, it takes longer. Sand is quick but creates heat and can warp quicker.
The most I have ever paid is 2000 bucks on a 49 caddy that had 1950's Gm under coating on the bottom, We spent 20 socking the bottom of the car in diesel just to be able to scrape the majority of the coating off. Some of the late 40's early 50's under coating is rock solid and 3/8 of an inch thick in spots.
6100 is insane but it sounds like he has not done any other blasting in a few months with all his equipment issues and has to make some money so It looks like he gave you the "PORSCHA" bill for blasting. Twice as much for a porsche part that fits a VW also.
Just buy your own blaster, That is what I'm doing my guy get 130 bucks an hour.