Best way to approach a good lease deal is not have a specific car in mind. Look for the best deal on a car you would be prepared to drive for the term of the lease. For a 2 year lease 25% of the price of the car is a good price, this is also usually based on the best avaliable purchase price from broadspeed etc. I've just signed up for a VW Golf gti, 5 door DSG in red, 10k mile per annum, £745.20 initial plus 23x £248.40, total of £6458.40, average discounted price for this car is approx 26k so lease is just below 25%
2 years is seen as the optimum time as no need for mot, prob nothing major other than servicing.
Take into account the any cost extras are paid for in full over the lease term, you always want to look for a car that has your required spec as standard.
The lease price also includes all VED charges for the term but not insurance. If the car comes with breakdown cover etc if you were to buy it, then you get the same when leasing.
My current lease Octavia RS tdi only cost me 5200 over the 2 years lease. Even if i'd bought it for 21k which is a very strong price, it's curent value at 20 months old is 14k, loss of 7k, so it would have cost me an extra £1800 to buy, without taking any finance costs into account.
IF you want a specific car, and want any optional extras, you're probably better off buying, if you're happy to be flexible in your choice and take the standard car without any extras, leasing could be right for you.