Hi zachneedmattress.
Welcome to our Mattress Forum! 
Polyfoam and Latex used as the support core in a mattress are not comparable in terms of overall performance. A latex support core is more durable, more resilient, more elastic, more adaptable to different weights and shapes and sleeping positions, more supportive (it has a higher compression modulus so it gets firmer faster with compression), more “natural”, and has a different more “springy” and responsive feel than polyfoam. Latex in general is a higher performance material and it is more durable than any other type of foams. As you discovered all these qualities also come with a “price tag” as latex is more expensive than a polyfoam core and for some people … a latex hybrid which has the benefits and “feel” of latex in the upper layers (the top 3" - 6" which are the most subject to wear and tear and contribute more to the overall “feel” of a mattress) is worth the cost tradeoff. For others it isn’t.
As all layers work together they lend their properties to the layers above and below and the feel of course will be different in a two-side mattress using polyfoam core vs. latex, and an all-latex will feel different if one sided versus two sided as well and you would be the only one to assess which is the feel you like best.
While price is certainly important of course … the “value” of a mattress purchase is what is most important and price is just one of many factors that can affect the “value” of a mattress purchase. There isn’t a “formula” that can be used to assess or “calculate” the value of a mattress because there are so many different variables and criteria involved that can affect the price of a mattress that can each be more or less important to some people and not to others who may have completely different criteria or definitions of “value”.
There is more about the 3 most important parts of the “value” of a mattress purchase in post #13 here which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists based on suitability, durability, and all the other parts of your personal value equation that are most important to you (including the price of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for).
Talatech can be blended or natural, the number ranges provided line up with what TG gives for blended (16-22 would be their 19 ILD, 21-27 would be their 24 ILD with “3 ILD tolerance”). Knowing blended or natural good to know if comparing to other items for value reasons. Polyfoam core good density at 2.5.
It seems to me that you are well on its way with your research and testing, and you are correct that having the complete set of foam specifications would give you more clues and help with finding the mattress that is best for your specific needs and preferences, but in the end nothing can replace own personal testing. The only way to know whether any specific mattress design or combination of layers and components is a good “match” for you in terms of comfort, firmness, and PPP with any certainty will be based on your own careful testing and/or your own personal experience when you sleep on it.
I am not sure what size of mattress you are looking at but at a quick glance here are some our Trusted Members here with all latex or hybrid latex systems that would fit your criteria.
• Luma Sleep with its all latex would fit your budget range and also an 11" that would fit your budget
• Arizona Premium would have a12" hybrid/latex and they can have multiple all latex options as well.
• Sleep EZ has also a few all latex options including Roma that would fit your budget and 8"[url= Compare Our Latex Mattresses | Explore Our Latex Mattress Comparison - SleepEZ]Latex Mattress Special[/url] that you might want to check on, it is a bit over your price range but you may wish to include it in your search.
• Sleep ON Latex also has a 7" and 9" Pure Green all latex Natural Dunlop mattress within your budget range.
• Latex Mattress Factory has also 7’ and 9" all latex mattresses within your budget range.
Some other members with systems that may be worth looking at in a slightly higher budget range:
• Foam Sweet Foam with a 7" All latex mattress.
• My Green Mattress with the [url= https://www.mygreenmattress.com/products/natural-escape?variant=17262282372]Natural Escape[/url] latex/hybrid line
Your assumption is correct but just in case you are considering going that route then whether you’re considering an all-latex product or one using an innerspring unit, I would still start your DIY research by reading option 3 in post #15 here and the posts it links to (and option #1 and #2 as well) so that you have more realistic expectations and that you are comfortable with the learning curve, uncertainty, trial and error, or in some cases the higher costs that may be involved in the DIY process.
Regarding component suppliers, while not inclusive, there are some options listed here.
I hope this will help you in your search and narrow down your options.
Phoenix