Prices for Randall mods are approx. 80-100 Eur (w/o shipment), it depends on many factors, mainly on elaborateness.
Cheap chinese amp sounds little bit sterile, but 4rockers preamp modification changes it to typical metal machine with good sound profile for typical metal backings. (v. 4/2016)
This cheap amp is too sterile for our taste. But we can make sound with more punch in the middle, better for metal players...