When you join the reserves you have to go through basic infantry training (you can't just go straight to Sandhurst like if you went into the regulars). You're paid £40.13 per day at the moment (I think I was paid something like £30 a day when I went through it a decade ago). I don't think anyone goes in for the money - I've made a days training sat on the toilet after lunch at my last contract gig.