In terms of parts, it's really quite reasonable. I put together a shopping list of all the components I'd need (although I actually had quite a few to hand already) from Bitsbox, and together with the PCB from Fuzzdog it all came to about 31 quid. I got the enclosure and footswitch from eBay. A bit of drilling and soldering and hey presto.
I subsequently made a 'Heffalump Junior' version in a smaller enclosure, with no blend or battery, and it was slightly cheaper at 27 quid.