Hey sweetandsourkiwi,
Sorry to hear about the foundation, not sure what there policy will be but it is not surprising especially given the weight of the mattress…although even the most basic box springs do not show that much failure at three months.
Good foundations can be a lot more than $250.
There is more information about the different types of support systems (bedframes and foundations or platform beds) that are generally suitable for different types of mattresses and some examples of each of them in this post. I would also keep in mind that these are general guidelines and some manufacturers have requirements that may be different so I would always confirm with the retailer or manufacturer of your mattress that any support system you are planning to use under the specific mattress you purchase meets their warranty requirements and guidelines.
Thanks
Sensei