Hi gustavus999,
That would depend to some degree on which mattress you purchased. some mattresses do much better and are more durable when used with a box spring (particularly innerspring mattresses that use helicals) and others do best on a foundation. You can read more about the differences between them in post #2 here.
As you are discovering … it’s usually less to buy a box spring at the same time as a mattress.
There is also the consideration that a box spring (vs a foundation) will also change the feel and performance of your mattress compared to how it feels and performs on the floor.
In any case … I would follow the manufacturers recommendations and warranty guidelines (which you can see here) which say that the mattress needs to be supported by a “matching” box spring so I would find out from them what they consider to be “matching”. If they allow for the use of other box springs or even a semi flex foundation … then it would be a matter of finding one that is suitable and better value.
If warranty coverage is not an issue for you … then I wouldn’t hesitate to buy a better value boxspring or a semiflex foundation (with some shock absorbing qualities to help protect your innerspring from damage) regardless of whether it nullified your warranty.
Phoenix