So, I attempted to replicate the issue, and this is what I found:
* When you go through the fare upgrade option, the screen does take you to the seatmap, which would allow you to figure out that you're not flying in first class.
* The flight you selected is on an ERJ-145, which does not have a first class cabin. The lower quote is probably due to the mixed cabin flights you selected.
* Agreeing that the final display is deceiving. Also, the weirdest thing is when I click on the flight details, the page place me back into Economy, with no option to upgrade.
The bonus 1000 miles is not worth the agonizing UX from Priceline.