Not quite expensive. You can get a pair from ebay for $70 if you get lucky. I have a set on my brushless emaxx and another set on my spare VBS just in case. The weight is comparable to the 6 spyders. I didn't notice any decrease acceleration on my emaxx .
Castle Neu 1520 on 6S LiPo Powered Gmaxx (Nitro Killer)
Predator with OS .21TM
Supermaxx with Mach .26
Revo with OS .18TZ
Kyosho ST-RR Conversion
Ofna CR with Tekin ESC/Motor (2)