Well to be honest - if you tried to buy the same exact hardware (say an i5/p55/4GB DDR3/GTX 460/60GB vertex 2E) in 6-12 months time from a retailer then the price will not drop much from what it costs today.
This is due to a few factors: The CPU, SSD, GPU and mobo will all be end-of-life, so buying these parts (just like buying s775 parts today) will not be simple, not much cheaper and not recommended.
However, if you aim to buy a system with similar performance, but not necessarily the same components then it is likely you will get it cheaper - how much is difficult to determine. Some things prices are unlikely to change - these include HDD, PSU, case, DVD drive and CPU cooler. RAM prices are difficult to predict - DDR3 prices could even go up. The only components that will get cheaper (ie next generation components with the same performance, but lower price) are the GPU, CPU, SSD and motherboard. For a £1000 gaming system you would expect to pay ~£600 in CPU, GPU, SSD, mobo. At a guess in 6 months time you will be able to shave ~£50 off this and maybe £100 in 12 months. However, prices are also heavily dependent on exchange rates, so this is no where guaranteed. Also VAT is going up to 20% in the new year.