Hi nasim20.
I agree with TMASC that you’d be hard pressed to find a good quality/value and “reasonable” mattress for the price range you are looking at, but there are a few better options that I’m aware of from our Trusted Members who have products close to your $300 budget limit that you might wish to consider. I am not sure if you are located in Canada or USA but at a quick glance here are some better quality/value mattresses (mostly innerspring, memory foam, and/or polyfoam mattresses). While I may have missed some I’ve also added some options worth considering with a $600 upper limit:
Christeli has one memory foam option
Sedona Sleep (slightly over $300) sells a polyfoam base mattress here
For around $200 Arizona Premium has a 7" of 1.9lb @ 31 ILD polyfoam base here and you can add some plushness with adding a separate topper.
Magic Sleeper has many low budget options mattresses that would fit your budget range.
Nest Bedding has their Love & Sleep Mattress.
Quality Sleep shop also has a good quality low budget lines.
Texas Mattress Makers here as some nice options and you may want to give them a call to see if they ship to your area.
www.addable.com/ also sells low budget memory foam mattress.
Some of the low budget mattress options in Canada (trusted members also) are listed here
[indent] Memory Foam Comfort has a high density foam mattress and cover for under $400 CAD
TMASC (The Mattress & Sleep Company) has Milly Tranquil lineup for lower budget ranges as well, but I am not sure if it comes in full bed size.
Some of the mattresses from Ikea (not members of the site) may also be worth considering (see post #3 here ~ and the posts it links to.
Just in case your personal value equation changes and/or you get lucky and have more cash available for this purchase, there are some “simplified choice mattresses” that are also in relatively low budget ranges (from $600 to $1000 queen size) are listed in post #2 of this topic ~that may also be worth considering and…there are also some great quality lower budget latex and latex hybrid options that are listed in posts #3 and #4 here ~ that are in “relatively” lower budget ranges as well (although these are definitely more than your budget).
If you purchase from one of our trusted members don’t forget to ask for The Mattress Underground discount see the (check the coupon codes here) as this may help a bit with the cost.
Best of luck with your selection, and let us know what you decide.
Phoenix