For a qualified contractor? $100 per billable hour is extremely reasonable. They're not on the clock all the time. When I did consulting work (not writing software but tech-related) I charged way more than that although it was usually based on jobs rather than hours.

ADDED: And, if you're contracting through a larger firm, they're adding various overheads associated with managing the project, etc.

They are standard rates for contracting and outsourcing firms.

500 euro a day is about 100k a year, accounting for 200 work days in a year.

Remove the margin of the firm, pension, healthcare, sick leave, company taxes, office costs, benefits... and what's left is the gross salary.