Misrule's mate is correct, you'll need to fill in the certificates of eligibility, send them to japan, get them back and then go to the Japanese embassy and drop the certificates, application forms and passports off for about 4/5 working days. I took my 11 piece band there a year ago and visa costs from the embassy were about 20 or 25 quid each. I think £425 a head at the promoters end for the entertainment visa is highly unlikely.
Don't go on a tourist visa if you can avoid it, they are very efficient over there and you may be sent straight home.
If you do go, bear in mind its very expensive so the promoter must provide everything..... prices can go up to £18 a pint !