Weight is the enemy of sensitivity, so heavy is bad, light is good. Which leads to small and light guides, because weight way out there on the blank has much more effect on sensitivity than weight in the butt. Small, light guides contribute not only to sensitivity but to balance because most blanks over about 6 feet are tip heavy.
Achieving lighter weight leads generally to manufacturers using higher modulus materials in their blanks, the higher modulus resulting in lighter blanks for the same power. Which leads to better sensitivity.
BUT. . . A very important ingredient in a really outstanding blank is design. For example one of my favorite blanks is a 7 foot medium light RX7 blank that costs well under $100. When I built one for my son his first reaction on fishing it was that it is "really sensitive." He uses it for tubes for smb, an application that I think it is a little underpowered for, but he loves it for tubes even though he has two SCV St Croix's of the right power. The finished rod weighs 3.6 oz, if I remember correctly.
How one determines how all this mixes together is not easy, but if I wanted to buy the most sensitive finished rod of a certain power and action, I would look to the reputations of the manufacturers and the highest levels of their rods. I know of no way to measure sensitivity. One thing you cannot do with modern high quality rods is evaluate them by twitching/wiggling them. (exc maybe fly rods) They don't wiggle with their hi mod materials and light guides. They leave many who are not familiar with this level of quality thinking they are too stiff. One thing you can do that will tell some of the story is to take a miniature scale to the store and weight different rods. Scales are cheap on Ebay or Amazon, and are very accurate. Weight doesn't tell the whole story, but if you're confused about the merits of one rod vs another, and one weighs in under 4 oz and the other is 5, it's very difficult to get a 5 oz rod (unless it's quite long or very powerful) to be really sensitive The old 7 foot spin Ugli Sticks weighed about 5.3.
I don't think it takes a retail price of $800 to deliver a great rod. I haven't been in the rod market (I build my own) lately, so am not sure what the top of the line (or close to the top) Loomis and St Croix rods go for, but I'll be you can get a great rod for about half that, or a little more than half.
The price-value relationship that we are used to in many products works for rods, too. Meaning that as you get pricier and pricier the incremental value added per dollar spent is less and less. Finding the sweet spot that meets your needs is the challenge.