keep in mind that expensive doesn't mean much. on mattresses. you need to look at them. if you go to costco, they were serta, a good brand.
i bought two from walmart, foam deals, not bad….
i sleep on a queen now, all foam, cost about $200..rollled up when i got it.
ALSO, did you get any maternity last night (clothes)
you don't need a box spring anymore if you use a plank. think about how you will make the bed, will you walk thru the two, or put them same plank..ask mat what he used…they have twins now