If you get the $45 prepaid card with 500MB of data, does that mean you get a fresh new set of data EVERY 30 days (whether you use it or not), or does your data usage continue to rollover and there is no way to get rid of it? For example, if I use 887KB of 500MB this month, will it rollover to next month's cycle or will data usage reset for me so I get to use a whole new set of 500MB? Does home WI-FI cost me data? How many KB's are in an MB? Thanks in advance....data confuses me.