The toll charges (for travelling on the peage) appear to be the same on either site (although I don't know what exchange rate the Sanef UK company will apply).
10 Euro to purchase the transponder + 1.5 Euros (for each month that you USE the tolls) from the French website Liber-t Balade
versus (from the UK website):
1.1. What does it cost?
Firstly, the tolls are the same cost that you would pay at the barrier. This new service simply allows you to use the automated lanes saving you time and reducing the hassle of manually paying tolls in euros.
i. Subscription Fees
a refundable security deposit of 20 (no TVA payable) for each Tag issued by sanef ("Tag Deposit");
the annual management fee of 6 + TVA payable in advance; and
a non-refundable application fee of 10 + TVA.
Total up front fee of 39.14 (including TVA) of which 20 is refundable when you return the tag
ii. Monthly Active Service Fee
In addition to the Subscription Fees, Subscribers must pay a monthly service fee of 5 + TVA for each calendar month in which the Subscriber uses the Liber-T System up to a maximum fee of 10 + TVA per year per tag. i.e. you only pay for the months that you use the tag and the maximum total Monthly Active Service Fee in any year is 10.
iii. Toll Charges
All toll charges incurred.
Finally, all fees and tolls will be collected in £GBP (see 1.3 below) by direct debit 10 working days after the electronic invoice is sent.