There is indeed an allowance paid, after phase one training, which equates to the cost of two car journeys to your sole primary residence per month. For Naval ratings it is not paid until you are on phase two training so I'm assuming for RM, it isn't paid before Ex Baptists Run. This being the case, your monthly outgoings upon joining must be less than 50% of your net income before any allowances are paid.
The allowance is called Get You Home (Mileage) GYH(M) and you either get Rail Warrants or GYH(M), not both.
The rules & regulations are quite involved, so it is not possible to give exact figures as every person, if they qualify, gets paid different amounts dependent upon where they live and where they are based. (No, it doesn't increase when you're in Afghanistan). The mileage rate paid per mile also varies for different length distances. Before anyone asks, I'm not going to attempt to calculate examples as it's something best advised by those qualified to do so once you join.
To qualify, you need to be able to provide evidence you are paying the mortgage on the property before you join& pay Council Tax as a primary resident/owner. If it's sub-let, you cannot claim.