No matter how much you spend, you will get 2-4x the use out of it if you buy a DOUBLE-SIDED mattress. Fewer and fewer places will even sell mattresses that are double-sided, and I think it's a racket. When you flip your mattress (about every 6 months), you spread the wear out across both sides AND both ends (put the head-end to the foot-end on one of the flips). Anything that has a built-in "topper" is absolutely limiting your use of the mattress to one position, and the topper likely won't last, meaning the entire mattress is ruined (I have family who spent a fortune on a quality mattress with a topper that couldn't be turned and in less than a year were stuck in the ruts they'd created). If you need a topper (and you might just need a softer mattress), I would buy it separately so you can replace it as needed.
Also, you can buy a mis-matched mattress, which just means that they were running out of the material or pattern used on one side and the other is different. These are "imperfect" and therefore sold at a huge discount, but the extra advantage is that if you get one that is softer on one side than the other, you can change your mind about how hard the mattress should be (i.e., flip it to the hard-side after injuring your back, and back to the softer side when you're well).
And finally, if you have neck problems from lying on your side, it might actually be your pillow that's the problem. They make really fantastic foam pillows that look like a wave, thereby supporting your neck and providing a dip for your head, keeping it in better alignment (they can also reduce snoring). Usually the wave on one side is higher than the other - I turn my pillow one way to lie on my side, and the other if I plan to lie on my back (the smaller dip).