I don't think that you'll save really much money in this case because a really good memory-foam sleeping stuff isn't usually a cheap thing. Of course you can find and get a very cheap one but such models lose their ability for changing and keeping shapes of your body positions rather fast and in the end they become very uncomfortable and almost impossible for using
Your main things in general while choosing such topper are:
- finding the one which will fit your bed perfectly
- selecting its density (it depends mostly on your body support and respond to its temperature too, the most popular type here is the density of 3 pounds https://sleepmentor.net/memory-foam-mattress-topper-reviews/ )
- selecting the thickness (it's important for your general comfort)
- considering the type of foam (gel-infused, natural or visco-elastic one https://www.foamnights.com/types-of-foam/ )
Also make sure that your topper doesn't have in its material any toxins and dangerous chemicals, for example in this case Ikea production isn't recommended.