Cambelt on a clio sport is a 5.2 hour job, check what labour rate the garage is charging and you can work it out from there.
Most garages won't fit parts not supplied by them as they offer warranty based on the fact that their suppliers warranty the parts supplied and will pay the garage for doing the job again if the parts were to fail in any way. If you supply your own parts can lead to a legal minefield if something went wrong as you would have to prove it was down to poor fitment or if it was parts failure.
Cambelt kit supplied and fitted £468 from me (serviceyourcar.co. uk ltd) job is done with renault genuine tools etc.
dephaser is about £150 or so on top if required although recommended as if it aint gone its probably gonna!
Breakdown of that is £260 labour, £130 belt kit, £78 vat.
Anyone doing it for much less than that is either in need of the money or isnt charging enough!
That should give you an idea of what you should be paying anyhows.