Is this a good idea?

I've been reading the average price of different types of cannabis in the uk on this website: 2005 Drug Prices for Cannabis - IDMU Ltd

According to this soap-bar is £9.45 of an eigth (3.5 grams). Using the purification technique could I buy soap bar and then purify it to get rid of most of the nasty chemicals and produce decent weed? Would this be worthy as it would be much cheaper?

You're not gonna be able to make 'decent weed' out of hash. You'll be able to make vaguely shitty hash, and you'd lose so much it weight it wouldn't be worth it at all.

Don't buy soapbar, it supports those who sell it...