I hope you don't mind a long post but I also proposed at WDW and I feel like if you can afford it no amount of $$ is too much for such a special event.
My plan was extremely elaborate, first because we visit WDW so often and my fiancé catches on to everything so quickly.
I had to think of a way to get her to WDW just a few months after we had just been there with family. At that time we had started checking into DVC memberships. So when we got home I found some Mickey stationary and sent a formal letter to ourselves saying we had won a 3 night stay with a special dinner one night a CRT at the MK. Plus the required tour of the DVC properties, etc. I wont go into all the details but it was pretty official looking and she fell for it.
We stayed at POFQ (on Disney's tab she thought) and on our 2nd night we were supposed to arrive at the MK in the evening for dinner. I had contacted
www.giftsofalifetime.com to help me carry out the rest of my plan.
When we arrived, i knew if I said I had to go to the restrooms at Town Hall she would also take the bait. When she went in, a Pixie from Giftsofalifetime was waiting for me with a rose and the first of 5 typed poems to lead her to her next journey. When she came out of the restroom she saw me standing there with a rose and I handed her the first clue/poem. It said to go to the wishing well, at the well she had to make a wish and then another pixie arrived with another rose and clue/poem to head to the next spot. We rode the carousel (She was on Cindy's horse of course-for those that don't know its the only horse with a golden ribbon on its tail) she read the next clue/poem and then on to Poohs (her favorite). There was a stuffed pooh waiting with another rose and clue/poem. And not to bore you it eventually lead her back to the wishing well where the last rose and a glass slipper/with ring was sitting on the well. I popped the question, we were lucky or maybe it was my helpful pixies but no one was around when we got to the wishing well, we felt like we were all alone. Afterward we had dinner at CRT and watched the fireworks from our table.
When we got back to our room my pixies again had decorated the room (rose petals, twinkle lights, champagne, the works.)
Wow what a evening...and best of all it was calling for rain all night and it stopped as we were on the monorail to the MK.
Some other day I will do a thread about what we are currently planning. I know this sounds crazy, but we are planning a surprise wedding (for our guest) at WDW. They will be thinking they are coming for a surprise 3 day weekend birthday bash!. I will let everyone know if we can eventually pull it off!!!
So an answer to your question...where to pop the question. Of course I would always choose the MK. But most of all make it personal and tie in what means the most to the two of you.
There is a spot in France at Epcot, where Belle and the Beast do meet and greets that has the stained glass panels that tells the story of Beauty & The Beast. See these 2 links
http://www.allearsnet.com/tp/ep/e_fran.htm &
http://www.allearsnet.com/tp/ep/shop7.jpg That could be a very romantic spot. Disney would probably help but I also highly recommend
www.giftsofalifetime.com whatever you can dream up they can probably help you pull it off.