Well for Janet's pink dress, I'd say the color is crucial. It's what stands out the most. Something that has a high neckline and zips up the back is also important, but I'd say less crucial. Sleeves are not important because she wears a sweater.
So here's a dress I found on eBay ($12.88). I picked it because of the color and because it was fitted on top and loose below the waist. The fabric is completely wrong, but that's not crucial. It's also too long, but it's easy to shorten. You don't even need to sew the hem, look for fusible hem tape. Glue it if you have to!
With some pink gingham fabric you can cut out a "Peter Pan" style collar and cover an existing belt. Hers is 1/2" gingham, but it's not a critical detail. Wal-Mart usually has 1/4" gingham in pink.
You could stop at that point, but a couple more details will finish it nicely. The easiest is the flower-shaped buttons that go down the center of her dress from her neck to waist (Wal-Mart also sells these). Just glue those suckers on. You can also add some white rick-rack down side fronts, and a big round slide-buckle to the belt. The easiest thing is to just have the belt close with velcro in the back - easier for the quick change after the proposal, and for clumsy Riff Raffs.
The sweater completes the look and you have a half-decent Janet costume in a pinch!
This top one is lacy, but has a great A-line shape on the bottom and I feel like the print is subtle enough to get away with. The gingham collar can completely conceal the scalloped neckline. It's $13.99 on eBay.
Another option on eBay (under $10 as of 7/9/17). The neck is a bit low in front - so turn it around and cut the tag out! The sweater will cover the back and nobody will know. This dress is knit, so there are no fitted darts to distinguish the front from the back.
