$600 would be high for my area, and I'm in a pretty expensive region.
IMO that adoption fee wouldn't be totally unreasonable for an English bulldog breed rescue, just because that specific breed has SO many health problems that their cost-of-care averages are much higher than almost anyone else's... but for a general rescue or another breed rescue, yeah, I'd probably look elsewhere.
$300 would be squarely in the middle for foster-based rescues in my area. That's a totally normal amount to ask here.
ARCHX TDCH Pongu the Insane, CD-C, RL1X3, RL2X2, RL3X (GSD mix, b. Apr 2010)
Crookytail the Tigerwuff, RL1, NTD (Akita mix, b. Jan 2011)