Making any home improvements can be an expensive outlay and for most people, they will want to have a clear understanding and idea of what kind of cost they are looking at before they even deliberate going for it.
For an aluminium bi-folding door which is standard size (3mx2.1m) you would looking at spending around £2200 for supply of the doors, with additional extras being added for installation. This is traditionally a set price unless there are difficult stipulations, such as on the 5th floor of a flat, which will incur greater costs. This price should include VAT but always remember to ask if this is included before you get stung at a later date.
Costs will dramatically decrease if your bi-folding doors are made with PVC instead of aluminium. However, if you are looking for greater security, aluminium is by far the better option and paying a little extra for this may be a better option. Aluminium also retains its colour better which might be another stipulation into how much you spend.
Although the cost of the door manufacturing is just over £2000, there are many additional extras which can be added which will inflate the price. These additional costs could include a differing in colour options. Although some could such as white and grey can be added as standards, if you require a more obscure colour, like a pink for example, these will inflate the costs.
Although many companies also include high level locks as standards some may charge more for snap secure. It is worth asking this initially as you do not want to compromise the safety of your own home just for £60 that you might have thought would have come as standard.
As afore-mentioned the delivery cost will raise the price too, as well as the difficulty of installing. However if the company which makes them also installs them, you do then not have to take any of the liability should it not fit (as long as they measured it too), or the installation is more tricky than they anticipated.
A choice of different glass can also raise the price of the bi-folding door, with high efficiency glass being more expensive than standard double glazing. Again consider the requirements of the room and work out if one or the other would be more beneficial.
To conclude, you are probably looking at somewhere between £3000 and £4000 for a standards bi-folding door, supplied, delivered and with your additional requirements added.