I know this is still going to be regarded as expensive, because it bloody well is, but Ocado of all places have maximuscle cyclone (chocolate) on offer at under £20 for the 1.2kg tub.
if you've never shopped with ocado before, you can get a plethora of discounts/cashback with them if your first spend is over £90 with them via topcashback
if you ordered, for example, 5 tubs, this takes you to £97.50. you can then use the £15 off voucher that topcashback give you and then they're offering £15.15 cashback on top, which brings this to £67.35, which equates to £13.47 per tub.
bearing in mind the cheapest place to get these, ebay, are around £30, this isn't a bad deal if you use cyclone
there's a £5 off your next shop voucher that can be generated when you sign up if you opt in for their marketing email. it can be used in conjunction with this, making it even cheaper