Whatever a Zetec turbo would cost to build, an ST170 turbo wouldn't be too far off that as it's a Zetec based engine at the end of the day. The limiting factor will be the ECU you'd use. You can't use/remap the ST170 ecu so aftermarket is your only option but the ST170 has VCT (which you can disable if desired) and a variable length inlet manifold, which you'd definitely want to replace.

The cylinder heads though are absolutely brilliant bits of kit and flow excellently for turbo-charging. The pistons, while decent quality, high-compression jobbies, have a question mark over them when used in forced induction engines so if you can get FRS rods + pistons you'd be sorted. You can pick and choose rods/pistons, head gaskets, inlet/exhaust manifolds to suit as they all interchange to fit.