Whilst there's a bare minimum rate, it can be negotiated, if you have the 'clout', hence some Leading Men/Women reputedly getting as much as £35,000 per week for their West End runs.
I would imagine that as a 'newcomer', the winning Dorothy would be on the minimum rate. Then, when her contract comes up for renewal, she (or rather her agent) will re-negotiate terms.
I just googled for general info and, as of 2007, the basic wage for a WE performer (not a lead) was approximately £19,000 per year.
Basically, if you want to make shedloads of money, don't expect to do it by working in theatre.