That sounds expensive, I paid 3 euro for mine last year, ready for a trip down france this year. The company I used links it to a UK bank account so no messing with credit cards and it takes payment monthly, so there is only one conversion fee.
Looks like the offer I used has gone, I use a 'liber-t evasion' tag, designed for occasional use.
http://www.atmb.com/en/the-autoroute...ber-t-evasion/