The 360 Arcade will not get cheaper. The Pro might.
They may choose to add "value" to the Arcade, but they will definately not make it cheaper. For one thing, they're most likely losing money with each arcade sold. If you think they can make a processor like the Xenon, a GPU like the Xenos, and 512MB GDDR3 RAM, with DVD drive, built in networking, AND a nice controller like the 360s for less than $200 (heck the retailers probably want a cut too, and you have to package it, ship it, etc.), you should go start up a "laptop for every child" program, and do some great things for the world. Microsoft's Games Division net profits dropped in the hundreds of millions of dollars this Holiday season, and they were wildly successful this year, compared to the last. I think the reason is obvious.
Also, if the 360 got as cheap as a PSP, or too close to the venerable PS2, people would start to wonder "Why? Why is it so cheap compared to the Wii and PS3?" "Microsoft is generous" wouldn't be the first thing to come to the buyer's mind, I guarantee.
No drop for the 360 anytime soon (which they have stated on several occasions). They will add "value", or drop the price on the models that actually make money (Pro/Elite). Sony and Nintendo still have the pricedrop card in their respective hands... MS played it last year, and now they've got to fall back on less powerful plays.