In reply to MisterPiggy:
I have always found Technique Extreme kit to be basic and good value, it does the job well. I only know their shop in Chamonix but there may be other branches. I have a pair of Decathlon / Simond trousers which I use for higher / mixed routes alpine summer ( and sometimes skiing too ) and they are great - really long lasting.
Obviously there are a lot of variables such as the routes you are likely to do, whether you tend to run more hot or cold, what the conditions / weather are like, what other kit you have etc. For lower routes ( up to about 3,500 metres ) and routes where I can move fairly quickly I tend to use lightweight trousers such as Montane. If it is a very cold / windy and early start I sometimes put waterproof over trousers on ( full length zip so quick to get off and no problems with wearing harness or getting over crampons ) but this is rare. However, for a lot of routes I find the Montane perfectly fine. On higher, more committing routes or where the ground is more technical / in the shade / mixed then I usually use the Simonds.
If I had to get just one pair of trousers and if the fit was right then I would have no hesitation recommending the Simond trousers. They do a lighter weight and a heavier weight version ( £39.99 or £49.99 ).