Play.com is the first online retailer to reduce the hefty asking price for Rock Band and all three instruments.
Its deal cuts the Band in a Box instruments pack - containing drum, microphone and guitar peripherals - down by 30 pounds to GBP 99.99.
Play.com is also offering 10 pounds off the game price, bringing the cost of the entire package down to GBP 140.
However, separate peripherals are being sold at the recently recommended price of GBP 69.99 for the drums and GBP 59.99 for the wireless guitar.
Elsewhere retailers GAME and Gameplay are yet to list Rock Band on their sites. HMV, Gamestation and Amazon are offering the game for GBP 49.99, but haven't put a price on the instrument pack.
Rock Band will be released for Xbox 360 on 23rd May. PS3, PS2 and Wii versions will follow later this summer.
Look out for our interview with developer Harmonix later today. It might just have the answers you seek.