Having just finished installing my Xpelair SELV extractor (through an existing airbrick above the shower/bath), I can add some input.
Fan was about £60, needed a roll of 1.5 Twin and Earth and a roll of Three and Earth from B&Q (found I had some T&E already, and had a lot left over) - cost about £10 for both I think, Fused Unit plus Fan Isolator about £15, they also do a 2 gang plastic moulded surface box which is divided into 2 separate single gangs (if you follow) which allows two units to be attached independantly for about £3 I think, a 20A 6 terminal junction box is about £2 but I already had one. So component cost was about £90.
As already mentioned by dingbat, a lot depends on access. I could access my loft space above the bathroom without too much hassle except I still itch from the fibre-glass I had to move out of the way - even wearing a boiler suit.
First I drew a diagram of the existing wiring to bathroom lights in the junction box on the lighting loop. I had a spare wire from my switch not being used - which was good as I wanted the fan independant of the light - but otherwise would have switched with the light.
Having switched the light circuit off at the CU, I removed the light and switch wiring from the existing JBox I took a length of T&E from it to the Fused unit. From this another T&E back to the new junction box and rewired the bathroom lights to switch (as it was originally). I also wired in the extra wire to a new triplegang light switch I had (extra cost would be about £8 say), but that was because I wanted it separate, otherwise could have just used the switch I had.
From the newly switched wire in the JBox plus Live, Neutral and Earth (or from existing switched live) I ran a length of Triple and Earth to the Isolator switch (Red=live, Blue=Neutral, Yellow=switched). From the other side of the isolator the three wires went to the SELV unit transformer. A piece of spare T&E without the earth finished off the transformer to fan wiring.
Switch on - all went well.
I had to drill some holes here and there, but nothing too extensive. I suppose it took me about half a day to complete.
I don't know what the hourly rate of an electrician is, but even if it's £20 ph, this works out about £100 say, plus some incidentals. So total cost should not be more than £200 - unless you have to knock out brickwork etc to fit the fan, which would be messy and add to costs a fair bit.
I tiled the wall over the airbrick (well extended some tiling I already had in place and fortunately had some spare tiles), leaving the correct sized hole for the fan in the tiles (oh, and also fitted a tight mesh grill over the inside of the outside airbrick to stop the nasty bugs flying in through the fan).
The use of the Fused Unit was to provide the requirements as specified by Xpelair - both live and switch fused. It also means that the bathroom light is fed through the same 5A fuse in addition to the 6A mcb in the CU, but I can live with that.
It can be done DIY, but if you are not happy or confident about electrics, then do get someone in.
