Latex comes in different types, from synthetic latex to pure natural latex (with 2 major types; Dunlop or Talalay latex). Latex mattresses also come as hybrids (coils) or all latex (no coils). So the price will vary with the type and style of the twin latex mattress. Starting with the synthetic latex twin mattress being the lowest to the pure natural latex twin mattress being the highest.
Foundations also come in different quality level and style which will fluctuate the cost.