TA Pay is based on Regular pay minus 13%, which is known as X Factor. The logic behind this is that a Regular Soldier is instantly deployable, whereas it takes a few weeks to mobilise a TA Soldier.
TA also get a Bounty (aka a tax free Bonus) if they complete all of their mandatory training. For an Independent Unit (the Majority) this is 27 days (including a two week camp or course) or 19 days for a Specialist Unit (including a two week camp or course).
Rates of pay and bounties for TA Commitment can be seen here (although I'm not sure that the pay rise has been added to the bounty rates):