I had my wife's Sienna done a few years ago in the middle of winter (shortly after we bought it) by Ziebart for around $200. They were the only ones that included powerwashing as part of the standard application procedure.
Krown Rust Control, Rustblock, and Rust Check are other popular rustproofing franchises. Krown offers a CAA discount. All are light petroleum-based products that creep into crevices well. Being light, they should also be re-applied annually and the salt spray can wash the coating off by spring. Some people have found that these types of coatings drip so you need to be careful where you park for a few days. The quality of work can vary from shop to shop.
Many shops use rustproofing systems that do not appear to have a web presence of their own. I'm not sure why this would be the case other than perhaps the chemical is produced by a small mom & pop operation. These types of rustproofing system include Canadian Rust Control and Oilskin.
For touch-ups, the DIYer can apply rust-poofing with an aerosol spray. Krown, Rust Check, and Rust Stop offer their products through dealers, retailers, or on-line. WD-40 will also work and may be cheaper.
In 2008, Erie Collision offered an $80 undercoating job ($5 off with a newspaper coupon). I got my 2003 Impala undercoated there for this price but it took some planning as they only do this on weekends (Saturday & Sunday) mornings in early fall and on Saturdays later on. It's best to call ahead since they stop undercoating for the day when they figure that they won't have any more customers. The ramp is outside so be sure to pick a windless day so more of the undercoating ends up on your car and not on the brush on the other side of the road. I had to give my car a good wash when I got home as there was a lot of overspray. I also noticed that the underside of my wheels got a good coat of overspray, which probably means that my disc brakes were undercoated too. I needed to get my Impala re-sprayed due to the inadequate work done by Erie Collision.
I also had to get a 2007 Dodge 3500 truck undercoated in 2008. Finlayson Tire does Oilskin undercoating and they had a $15 off coupon in the newspaper. The truck cost $130 (because of its size) and this work was done in one of the bays in their shop. Very little overspray and they cleaned the overspray off the glass when they were done. My 1973 Dodge Dart has an Oilskin sticker on a window and the undercarriage and doors are all still in excellent condition even though it was regularly winter-driven so I expect the truck to last a long time too. However, after a closer inspection, there were many obvious places (like the hood and front fenders) that were completely missed. The technician also appeared to go around the vehicle in one direction as many places were missed because he probably didn't look backwards as he was working. The moral is that the quality of the job depends on the skill and experience of the technician. Check the work afterwards to make sure that you got the protection that you were promised.
Lately, I've been getting my cars rust-proofed at the local Krown Rust shop. The work appears to be thorough and the price is competitive.
Many independent shops provide undercoating work as well. Often, they will use an undercoating product (like Kleen-Flo Rustproofing or Dominion Sure Seal Anti-Corrosion Material) from their usual autoparts supplier. The important thing is to find a shop that has the skill and experience to apply the coating correctly. Being independent, these shops can offer a complete rustproofing job for around $80 since they don't have to pay for franchise fees and specialty products.