I have just done a 'reverse engineer' of the sample Audi Solutions quote I did yesterday with my dealer and the Final Payment works out as 42.4% of the original purchase price after 3 years and 36.2% after 4 years. The residual value may be higher but what figure are they taking, dealer selling price, private selling price or trade-in value.
I have had eight A3s so far and I have had the last three of them for 2.6, 2.4 and 2.8 years and have got a trade-in value of 54%,52% and 53% respectively so 50% after 3 years seems a reasonable figure for them to take. The difference between the residual value and the final payment will be the amount you have available to put towards the deposit for the next car.
As you will always be paying a percentage (currently 6.6% apr) for the finance I cannot see how it can be cheaper than using cash unless you can find a way, in the present financial climate, of getting more than 6.6% apr on your cash. If they can tell you where you can to get a risk-free investment that currently gives this sort of return, I for one would like to know about it. The best I can get at the moment is 3% on the first £5,000 in my current account.