Here at Tres Amigos RV Park on Stone Island/Isla de la Piedra rates for one week start at about $120.00USD per week. You can stay a month for $350.00USD. Front row sites are full and are more expensive. You can be in Centro in less than 15 minutes.
Photos are available on my blog.
We have just down your route in reverse. Enter at Lukeville and get your passport stamped, go to RP and enjoy your visit. You can either come back via Soynata or proceed to Carboca, actually is is Pitiquito for your car permit. Details on my blog. Bur Les...if you are going to be living full time in Mexico there are different rules. Check out http://yucalandia.com/driving-in-mexico-issues-fun/current-aduana-car-import-rules-made-obsolete-by-inms-new-residency-categories/
We strongly suspect that 54 litres of diesel fuel were siphoned off our RV while we slept in the Los Mochis RV Park. If not then, that means that the last Pemex at Soynata has a crooked meter. Details on my blog if you are interested. We are 99% certain it was not the meter at the Pemex