A topper won't make a soft or unsuportive (ie knackered) mattress any better. It can make a too firm mattress more comfortable. I wouldn't spend huge amounts on one...you'd be better to get a new, satisfactory mattress with the money.
I have an old memory foam one, but can't remember the price, sorry. Not a fortune or I would have remembered ! It's lovely and comfortable, but I take it off in the summer as it gets too hot !
Only experience I have is sleeping at a friend's house on (I presume) a feather filled topper. I didn't really like it & if I was going for one I would think about foam/memory foam.
I’ve done the topper route many times and prices. The best will last for a while, but eventually a new mattress will have to be purchased. Bite the bullet, get a new decent mattress.
I use a M&S one....a duck and down one and it's certainly lasted and been very comfortable, £109 for a superking sized one, cheaper the smaller the bed.
We bought a mattress with a built-in topper (memory foam) - never again; the mattress is still OK but the topper is knackered so we've ordered a new mattress.