I would also like to add that you could contact the sub-contractor, enquire as to any discount they will offer you for this job and mention that you will continue to use them for other up and coming jobs - they should give you a good discount eventually but don't expect too much so soon. With any discount they give you you then add that back onto the actual figure they would've charged you - using that as your profit.....remember, don't be too greedy and expect too much profit, you're not the one doing the work since you are sub-contracting out, any profit is better than none at all and your customer should not be ripped off!
As jimmyjames pointed out - do make sure they have all the necessary H&S and method statements etc. - if anything happens and you didn't make sure of the necessary and legal requirements you'll be in serious trouble!