too much is relative to... how much money you want to spend....
but here is my 2 cents about a pricing range. (Disclaimer: not what i paid for mine.)
1. Red version, astro zombies exclusive = $175 - $225
2. Glow vinyl with silver spray = $150-$175
3. Glow vinyl with blue spray = $150-$175
4. Grey vinyl, Pushead Fan Club exclusive = $75 - $125
5. White flocked version, WCC 21 (December 2005) = $175-$225
6. Hand Painted flocked version, various colors (December 2005) = $400 - $1000
7. Black vinyl with white head and red star, Osaka Superfest (February 2006) = $50 - $75
8. Black vinyl with green head, Osaka Superfest chase version (February 2006) = $65-$80
osaka screwbiter skullbrain $125-$175 (give me yours)