My recent experience...
Very few people are buying second-hand cars right now, which means used prices are terrible. Your trade-in price reflects this. You'd be lucky to sell your car privately for good money. Why not play the market, cancel your order (as per above posts) and go for a nearly-new A5 instead?
I tried to trade my A4 in for a new A5. I was quoted 10k one month, then when I had my finance together and returned a month later, the trade-in value had dropped to £7.5k. I was NOT happy, so tried to sell my car privately. I wasted two further months and the cost of the adverts, as the highest private bid (even on ebay and autotrader) was £5k.
Now, knowing that the bottom has fallen out of used car sales in the UK, I guessed that the Audi garage would be finding it (relatively) hard to shift second-hand A5s. I would get less for my trade-in, but I thought I would be able to drive down the price of a nearly-new car. So, I ignored the trade-in price discussion, and focussed my negotiations on the price to change. It worked! After a couple of days of telephone negotiation, I got a cracking deal (10k on the clock, 9 months old 3.0Tdi, 19" wheels, etc.): price to change £20k (that's around $35k dollars US I think).
I am now the very happy owner of an A5 that cost considerably less than I was expecting. I've spent the difference on a new bathroom!
Don't get pressurised into buying a car (even the mighty A5) for the wrong price right now. It's a buyers market.
Potemkin