My implant just screw fell out. What would be the cost if I still have the post of reinserting?

if the healing screw fell out, it could be places back on the implant by the surgeon who placed the implant. Usually at not cost It is a time sensitive matter since the gum tissue could regrow and cover the implant, in which case you will need to have the exposure surgery again See your surgeon asap 

I am sorry to hear about your situation. Unfortunately, it is not possible for me to give a cost without knowing the brand of implant that was used.

As long as it's just a straightforward situation, the fee typically would be similar to recementing a crown. Usually around $100-200. If you can see the person who placed it, there isn't usually a fee involved.