When purchasing a mattress don't always go for the cheapest price but always go for quality even if it costs a bit more.
If you spend a bit more money now then you'll have a quality mattress that will help you get a good nights sleep.
I have and recommend latex based mattresses which have lasted me the longest.
I've had my latex based mattress for 12 years now and it is still holding up really good.
Regular memory foam mattresses are pretty long lasting as well and they tend to last 8 to 10 years.