Hi Shazbot,
I don’t know what would be the cheapest out of all the suitable options that are available to you but if you choose to go with a wood slat foundation then either the bedlegs or a steel bedframe would probably be among the lowest cost options available (there are some relatively inexpensive bedframes as well that would be in a similar price range to the bedlegs).
There are also some steel wire grid type platform bedframes listed in the foundation post here that would be a lower cost option yet and if you add a bed rug to them to even out the support surface (see post #10 here) they would be a suitable choice as well for an all latex mattress.
Phoenix