A lot will depend on the type of open sight you have in mind and the available metal on the gun you want it mounted on. Also, make sure you engage the services of a qualified gunsmith. Someone you know has a reputation for doing this kind of work andd doing it well.
Yes it can be done. But impossible to price. You left out too many details.
What type of sights do you want?? And what type of rifle is it??
If you just have a standard bolt action then the cost will not be that bad. If you have not bought your sights yet. Then advertise on a few forums for a set of Remington 700 BDL take offs. Those were some very good open sights. And there are alot of them laying around that someone will sell you for a reasonable price. There are a bunch on E-Bay.
But if you want an aperture for a lever action then that can be a different story. Tom.