Washable breast pads will save you money and will help save the planet too.
I suggest that you need at least 18 pads (9 pairs), so I have costed them accordingly: Boots sell washable breast pads - £5.49 for 6 pads (so 18 pads cost £16.50, or just £11 when they are on the 3 for 2 deal). The reviews for the Boots washable breast pads are patchy, but I found them excellent - great value yet long lasting, and very importantly, they have a plasticky-waterproof layer to stop the excess milk reaching your clothing.
I have tried (more eco sounding) fully cotton and fully bamboo versions but they just leaked.
Other brands making waterproof reusable breast pads are: Close Parent - £10.99 for 6 pads (so 18 pads cost £33) and Pop In - £9.50 for 6 (so 18 pads cost £28.50).
All washable breast pads come with a small mesh bag to wash them in. You MUST use the mesh bag EVERY TIME you put the pads in the washing machine. In the early days we had to call out a washing machine engineer to unblock the washing machine (not idea l when you have a tiny, mess-generating newborn) due to a breast pad in the pipework...oops! I find zipper mesh bags easier (and more relaible) than the supplied drawstring ones, they are also useful to put the baby socks in to stop them blocking pipes too.
If (like me) you started off using disposable pads (and began despairing at the waste of money and the excess of plastic), then give reusable breast pads a go.