As with anything in life, the answer lies in common-sense.
A $30 mattress is hugely unlikely to provide you with the support you need.
Although it is true that a mattress topper can rejuvenate an old bed, it would be a bit foolish to put a brand new high quality topper on a terrible mattress.
It's like buying $4000 tires for a $200 car...
http://www.mattressma..._mattress_topper.aspx
Could get one of these as they're relatively cheap, but a great quality.
What do you think?