I'm selling an Old Tau Army (Warhammer 40k) and need a rough value?

As long as the paint jobs are at least decent I'd say at least $300 based on the retail price of all those models and materials. Of course it will also depend on demand in your area. It would be best to go to your local comic/hobby shops or Games Workshop store and ask around. Put up flyers (With pictures of completed models) if they let you. $300 or best offer would be a good place to start I think.