Go on John Lewis website as this will give an indication of the 'off the shelve' price. This is aound an average then you can work upwards ££££
Most fabrics are over £20pm. What interior designers do is make off the fabric prices and charge you a fixed price on the making. ie they would charge you £30pm when you could source at £25 so already they have made £5pm then say charge you £50 per curtain (wow, then you think thats good).
Some good makers (that advertise in the local press) will use your shop bought fabric. Its worth sniffing around e-bay to source off cuts from good designers like harlequin or prestigous.
I had a harlequin roman blind made on a 1800 x 1200 window which came in at £250 each. I think the standard patio window was around £200 ish (plain no pattern match). On top of this you need to add on poles or metal tracks.
In hind sight i would of had all mine interlined as the lining is thin. Watch out for pattern match which adds price. Use plain fabrics with patterned this also cuts down price.
Its a pricey business and you need to make sure you love the fabric before going for it.
You can get blackout blinds from Homebase, B&Q and Ikea.
As for curtains, the most flexible way to get them "in blackout" is to have blackout linings on the back. You can buy these from John Lewis, etc, or else get blackout material and stitch curtain heading tape onto the top of them (this will make them the same as the JL blackout linings). Then use curtain hooks to attach the linings to the curtain heading tape on the back of the other curtains, one level below the curtain hooks which will hook to the curtain track.
NB - this is for curtain track, not poles. If you have poles, you will have to add curtain RINGS, then hood the curtain hooks to them.
Swish hooks are the best for the (curtain track) system I described above.