PayPal offers 3 integration types:
1. Express Checkout (the most common)
2. Adaptive Payments
3. PayPal Payments Pro
You're trying option #1, which lacks support for variable pries.
Both Adaptive Payments and PayPal Payments Pro offer recurring charges with dynamic amounts, but require PayPal's approval and quite a bit of paperwork. If you're approved, the APIs are pretty simple in both cases.
May I suggest some easier alternatives?
1. Use Recurly who are already approved for #2 above (adaptive payments). Other good alternatives are BrainTree and Stripe.
2. Change your pricing policy to a monthly charge of $X, granting a set of tokens. The user can upgrade to a higher plan to get more tokens every month.