pre-authorisation not debit
From memory (having used the Velib bikes on several trips to Paris), the scheme there takes a €200 pre-authorisation, it doesn't actually charge this amount to your card. I certainly don't remember my account being €200 short for a month, but then again I was never that skint that it would have really bothered me.
Re the hourly rates, with the first 30 minutes free, I found that to be half the fun as a tourist. A fun challenge was to find the next Velib station meeting all of the following criteria:
a) has a free slot to park your bike (can't hire a new bike until you return the first one)
b) has a free bike
c) has a free bike with air in the tyres and functional brakes
d) is closer to your ultimate destination than where you picked the bike up.
About the only time we failed at this game was on the final day of the TDF, trying to get close to the upper turn on the Champs Elysees. We ended up riding around in ever increasing circles finally finding a station with vacant slots a couple of km away.
In 6 days of Velib hire, I think we only paid the €1 hourly charge three times.