I have the attachment and yes you can earn good money doing it. It is a good add on but not my main business.
If you want to do drains full time then it is worth investing in the right kit (lower pressure, more flow rate).
It can be a bit smelly, but that service commands a good price!
You need to realise though that the drain attachment is not designed for anything other than household drains... anything bigger and you won't be able to do the job! So careful what you take on.
Dynorod are offering a 'fixed' price at the moment of £99... a bit misleading as it is only fixed if the job takes 1 hour or less... anything over that and they charge more!
I did a drain the other day and charged them £60 and the job took me 20 minutes... good money in my book... the customer was chuffed to bits because I saved them nearly 40 quid and I was happy 'cos they paid me
Good luck... hope this helps
Andy