Hi mccarthd26,
Since it appears that your in laws will be the most common user of the mattress … if I was in your shoes I would find out which Tempurpedic model they have and look for a similar memory foam mattress from one of the manufacturers in the online list here that was a close comparison to the one you already know they sleep well on and was as close as possible to your budget range. If it’s the cloud luxe then it would be on the soft side more than a “middle of the road” side so you would need to decide if that was OK with you (it may be too soft for some of your guests). Otherwise you could choose one that was a little firmer (and likely less costly). Other than choosing a a mattress that is close to the one they have … I would use your phone conversations with each manufacturer as a guideline about which of their mattresses they think would make the best choice for your budget and an “average” comfort range.
Some of the better options I’m aware of in the Denver / Boulder area are listed in post #2 here. Since the mattress isn’t for you I would do most of the work on the phone since testing a mattress wouldn’t be necessary. If you wanted to take the time you could also test the Tempurpedic mattress that they have in a showroom and use your own “proxy testing” as the basis for deciding on a mattress which had a similar feel and comfort/support level but it may be difficult to find something that was close to their mattress and still had reasonable quality memory foam inside your budget.
One other option that may be worth considering is a basic firm innerspring mattress and adding a memory foam topper (around 3" or so) so that your guests had the option of a firmer mattress or a softer mattress with the memory foam topper (along the lines of post #2 here).
Phoenix