its not just buying and fitting the vauxhall (or whichever) engine............its the cost of the bespoke subframe that you have to use.......and all of the other bits that you have to change..............and after all that you will need to take a morgtage out to insure it.......and the insurance company will want to have the car checked to make sure that its safe and that the conversion has been carried out correctly before raping a load of money from you.
in other words...................dont bother...... 
Not necessarily true.
If you are really careful, get a cheap donor (talking £200 max) and are prepared to build your own frame and shafts then I reckon it could be done for under £1000. My sub frame has cost less than £100 including the rubber mounts although I did have some of the steel already.
You need to be able to corner weight the car properly and set the suspension up so you will most likely need to buy aftermarket suspension parts that are adjustable. Most people complain about the way they handle due to the suspension not being properly set up.
As for insurance you will need to look at specialist. Most main stream insurance companies will charge you a lot but the specialist will give you a better deal. When I was 18 I looked at what the cost would be on my Vauxhall conversion. I was managing to get quotes of £1200 where as my astra at the time was costing me £800. The quotes I got included all the modifications I was going to make (brakes, roll cage, bucket seats, engine etc) and an agreed value of £5k.
As for your insurance company wanting to have it checked. Those that do ask will ask for an engineers report. This is basically a piece of headed paper from a garage saying that the conversion has been done properly. My local classic car specialist said they will do one for £20 and will take about 30 minuets to do.
When I did the auto to manual conversion on my mini no-one asked.
As for work and time. I have so far spent over 4 years on mine although that has included a full resto. So far I have a shell with a roll cage in it and the engine sitting in my home built frame. Bucket seats are mounted, pedals, steering and one side front suspension. I have a load of parts painted waiting to be fitted and that is about it. A pre built kit will be quicker but a home built frame will be cheaper. It depends on your skill level. If you are going to build your own frame then you need to be able to weld and have good fabrication skills.
Hayling over on 16v miniforum did prove that if you have a long weekend (4 days) then you can pull off an engine conversion but he is a very experienced car builder. I reckon with my experience (3 years mechanical engineering degree, 1 year Automotive Msc and about a dozen like for like engine swaps on cars) it would take me about a month assuming I had the money, all the parts and nothing else on.
