Hi RoboLobo,
Post #2 here talks about the different types of foundations that may be suitable for different types of mattresses. There are no hard and fast “rules” but there are general guidelines that are usually best to follow. The recommendation of the mattress manufacturer or the matching foundation (if there is one) is often if not usually a good idea to include in your purchase.
The foundation thread here also lists many different types of foundations with many links to examples of each.
If you are considering a KD type of foundation with an all latex mattress then the slats should be a maximum of 3" apart (preferably less) and most of them are more than this. For a polyfoam base … then the slats can be further apart because polyfoam isn’t as elastic and is stiffer than latex. Most of the KD foundations are less costly than other slatted foundations that are suitable for a latex or other “all foam” mattress.
Depending on the mattress … “cheaper” foundation you are thinking about may or may not be suitable depending on how it is made or the mattress that is being used on it. In many cases good quality foundations are available locally at good prices but it’s important to know what you are buying because there are also lots of poor quality “junk” foundations available that can cost you more in the end than they will save you.
One of the links in the foundation thread are to a post where one of the members here built their own and provided the design they used and instructions for materials and construction for those who are more interested in a DIY foundation. Good support that is appropriate to the mattress is important though and is a small price to pay for a better performing and longer lasting mattress.
Phoenix