Hi WildStringdom,
It’s a little difficult to find good quality innerspring mattresses online because there are usually many available locally and shipping costs can raise their prices significantly (they usually can’t be compressed for shipping with courier). Most of what you find online will usually be mainstream mattresses where it is usually very difficult to find out the quality of the materials that are inside the mattress. There are also so many different types and styles of innerspring / polyfoam traditional mattresses that it would be difficult to predict which one would be suitable for you without trying it in person and in most cases if you don’t make the best choice there can be substantial cost and difficulty involved in returning a whole mattress.
Some of the better options I’m aware of in the Washington area are listed in post #2 here and there is also a Baltimore list in post #2 here although most of these are duplicates.
You are probably looking at a traditional mattress with polyfoam and a good innerspring in your budget range. In a two sided construction I would look for at least 1.5 lbs polyfoam and in a one sided construction 1.8 lbs or higher would be a better choice.
You can read a little more in this article and in post #10 here about the different types of innersprings. Pocket coils are more conforming and have better motion separation than other types of coils and often need less foam in the comfort layers while an offset coil or even a Bonnell system can be stronger and a little less costly but often need more foam above the coils because they are not as conforming. In the end it’s a personal preference and a mattress that provides the best PPP (Posture and alignment, Pressure relief, and Personal preferences) would make the best choice regardless of which type of innerspring it has inside it …as long as the comfort layers are good quality because they will be the first to break down.
Phoenix