Rate: R
working hours: 8
working days per month: 22.5
Mutiplier (1.5-2.5): 2
monthly income: 7000
7000 = R x 8 x 22.5 /2
or
R=7000/8/22.5*2
The mutiplier=2 is normal, depends on the work and the client
This is only the salary part. If you need travel and other expenses, that is direct cost, no mutiplier is required for direct cost!
This is an estimation. You should estimate the cost of vacation and sick leave or other reasonable cost (there could be someway to estimate this part for cotract work).
CCC's estimate is ok! I believe asking 80-100$ is normal (and no overestimation at all)