Wm is quite right about the leak. I was a mechanic for 7 years in the Army and I've been using those skills for the last 30 years. Even with the great big oxes in the Army and the hammer happy mechanic, I've never seen one broken. They may look fragile but they have to withstand a fair bit of pressure so they are pretty strong.
As for what garages charge
I was charged £400 to free off the brakes on my bike for MOT. I let the bike shop do it because I simply didn't have the time or space to do it myself (I have the tools). When it came o the rear caliper I did it myself, cost a bit of dirt, some WD40 and a few bits of skin....
With my Galaxy I was charged £700 to replace the back brake disks (not pads note) and that was Jersey with no VAT. When I needed to replace the front one's I bought a complete set of disks (4 for the whole car), with pads and fitted them myself. Delivered by Ebay, £100.
My Uncle tends to get his Citroens serviced with us in France. With costs going over £100 per hour for Labour it pays a large part of his travel costs for his holidays in Portugal as our labour rates vary from €34 per hour to €41 per hour and that's a main dealer.