Depends on age and qualifications. Certain parts of the country are better, experienced staff are paid more. You are often also housed, which can vary from quite grotty to rather lovely, and makes a huge difference to your net earnings. Plus winners bonus' and pool money, which is a share of the yard prize money. Probably the best paid of the horsey jobs, as a generalisation.
I was working weekends at a smallish race yard last year...just yard work and earned about £7.50 an hour, BUT, it tends to be spilt shifts,so unless you live in or nearby, it may mean some hanging about and can take up the whole day although you only get paid for the 5/6 hours you work