Mobile data is still the most expensive part of mobile phone service in this country (especially on prepaid PAYGO), and there are ways to easily gut mobile data usage, especially if GPS is being used regularly (note my posts from
this thread and
elsewhere). You even
halve that 500MB data requirement, and if T-Mobile coverage is acceptable, you're already down to $20 on P'tel for an overkill "unlimited" talk and text plan. Get that data under 100MB, and a PAYGO plan could work for around $10/month or less.
Read the section on
cellular providers from the guide if you haven't already. P'tel, Airvoice, or TruPhone could all work, potentially.
The only catch with wanting SIM cards and phone numbers to stay active long term with a cheaper prepaid plan is needing a long-term PAYGO plan that keeps airtime active for the gaps needed. Many have a 45-90 day no-use account lapse clause that can basically eat any remaining balance if they aren't used, so read the legal agreements. Truphone's the only one of those mentioned providers that can be used outside the US to do that with. Otherwise, you'll have to treat the SIM cards and the money loaded as disposable if the out of country gaps are too long to keep the balance and number active with the provider and just keep spare SIM cards for activation as SIMs can't be reactivated once an account is closed out.