There is the St hybrid root. There is the fullley forged root. And there is the Rs internals root.
The price will vary wildly.

Ive done the St root and am building one with the Rs parts root.
For 300bhp with a bit of head room the Rs rep is not a bad way to go.

Rs pistons £300
Rs spec forged rods £330
Piper valve springs £200
King bearings, oil seals. Bolts and gaskets £200
Engine machine work ( deck & hone ) £100

Add another £600 for forged pistons rods and machine work.

will you build it your self or pay an engine builder ?

The engine is the cheap bit