Day 9 of the 12 Days of Deals on Xbox Marketplace, sees a 60% off a whole stack of Sonic hits:

Sonic 4, Episode 1 – was 800 MSP now 320 MSP
Sonic Adventure – was 800 MSP now 320 MSP
Sonic Adventure Upgrade – was 400 MSP now 160 MSP
Sonic the Hedgehog 2 – was 400 MSP now 160 MSP
Sonic the Hedgehog – was 400 MSP now 160 MSP
Sonic the Hedgehog 3 – was 400 MSP now 160 MSP

This is a one day only sale so get on them at super sonic speed.