First MTB? how much to spend?

My brother & a mate are looking for their first XC mountain bike, how much should they spend? I told them £300-£500, but they seem to think the Argos BSO's are good enough, how can I get them to understand that in order to get a usable machine they would have to spend atleast £300, I let them use my SS Hoodoo & Whyte 801 & my XTR Bizango. But they still don't want to pay £400 "when I can get a £120 bike from argos" should I let them get a cheap BSO & watch them suffer or try to get them to understand?
Bikes:
2013 Mondraker Dune XR - XX1 XO1 Hope, Easton Carbon, Fox
Voodoo Bizango XT - Hope, Pikes, Mavic
2013 Mondraker Dune XR - XX1 XO1 Hope, Easton Carbon, Fox
Voodoo Bizango XT - Hope, Pikes, Mavic
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Posts
http://www.southcoastbikes.co.uk/Dont-Buy-a-Cheap-Bike
http://www.londoncyclist.co.uk/what-happens-cheap-bike/
http://ozsoapbox.com/cycling/5-reasons- ... uffy-bike/
There is no secret ingredient - Kung Fu Panda
London Calling on Facebook
Parktools
Then bought a low end proper mountain bike which was much better and lasted for years until i got a decent bike.
2013 Mondraker Dune XR - XX1 XO1 Hope, Easton Carbon, Fox
Voodoo Bizango XT - Hope, Pikes, Mavic
There is no secret ingredient - Kung Fu Panda
London Calling on Facebook
Parktools
2013 Mondraker Dune XR - XX1 XO1 Hope, Easton Carbon, Fox
Voodoo Bizango XT - Hope, Pikes, Mavic