Cos you read somewhere that this is the price of a windscreen, must be true then. Think you'll find it's a bit more than that.
I think fitting a windscreen is a bit more complicated than changing brakes pads, disks and engine oil.
The advice you have been given by members here and an actual professional windscreen fitter who does this for a living day in and day out and who's opinion should be taken 100% is do not do it yourself!
You've seen a video and you've changed brakes and oil so that qualifies you does it? I don't think so mate. What if you don't fit it correctly? Huge possibility of this happening as you've never done it before. For all you know the screen could blow in while your driving down the motorway or fall outwards under heavy braking.
There have also been a few incidents of badly fitted windscreens on an IS leaking even when done by the professionals. Then water getting into electrics and causing major headaches! Thing is if it's done by a proffesional company and it leaks then you have a guarantee and they are liable. Do it yourself and it leaks and screws the electrics, all you've done is create more problems for yourself!