Wot ? 500 quid ?? They throwing in a whole new chassis for that or what ?
If you've got access to ramps, a compressor and a Schutz gun, you can do the job yourself.
It's not regular waxoyl you want though, it's not all that durable, especially with our roads and weather and it's really only good for door internals, inside chassis members etc where it isn't subjected to the above rigours.
What you want I should something like Hammerstein Schutz (which has got waxoyl in the mix) and a litre will do the underside no bother at all (ok so that's only one coat, but at under a tenner a litre, a couple of coats is the business.. under 20 quid well spent IMO (providing you have ramp etc access. You can get it as a brush-on version, bit messy but very good. Around 15 to 20 quid for 2.5 litres depending on where you buy it. No bother doing it especially if you can blag the use of a garage with a pit or better still, ramps.