Straight from their website - it's as clear as mud:
How much does it cost?
The Tag is free, but remains our property and the toll charges are less than they are for cash. We ask you to make an initial payment to enable you to use your DART-Tag immediately, followed by payments the amount of which you choose. The initial payment is not a deposit. It will be credited to your account and toll costs will be deducted from it, as you use the service. Your account must remain in credit at all times or the tag will not work.
How should I calculate my initial payment?
However you choose to pay, ensure your account always remains in credit. If you elect to pay a fixed amount each month by direct debit your initial payment should be sufficient to ensure your account remains in credit until we can collect your first monthly payment. Suggested payments for each class of Tag are shown on the application form.
How should I calculate my monthly payment?
However you choose to make your payments, they should always be sufficient to ensure your account remains in credit. If you have elected to pay a fixed amount each month, a calculation based on the average number of trips you make each month multiplied by the toll rate for your vehicle will be calculated on the application form. If you require more than one Tag on your account the calculation will be made for each Tag and and a total given on the application form.