This probably belongs down in the larger scale forum but the quick answer is, the HPI RS4s are a good chassis, pity they’ve had 3 versions, in 1/10 electric, 2 versions in Micro RS4 and a few more in Nitro RS4. In other words it’s pretty confusing and a ‘cheap’ one could be pretty poor.
HPI made so many different ones called “RS4” that its not uncommon to find used ones in the 2nd hand rack for under $200.
Common ones are:
1/10 RS4 (the 1st and original 1996 car)
1/10 RS4 Pro
1/10 RS4 Sport
1/10 RS4 Pro 2
1/10 RS4 Sport 2
1/10 RS4 Pro 3 – latest, comes in various specs
1/10 RS4 Rally
1/10 Nitro RS4 (gas version of Sport)
1/10 Nitro RS4 Racer
1/10 Nitro RS4 2
1/10 Nitro RS4 Racer 2
1/10 Super Nitro RS4 (SuperTen size)
1/18 Micro RS4
Pro 3 was the first all-new redesign for 2000, otherwise all the RS4s (except the MicroRS4 onviously) shared many common parts.
Snooping around the tracks, its quite easy to find MicroRS4s being sold 2nd hand for < $200 too.