Stuff I've read seems to puts windows lower down on the list of priorities in home energy efficiency measures
think mainly due to cost , so longer ROI in cash terms , money better spent elsewhere first to give greater reduction in energy use

As you've mentioned before draught proofing windows is a higher priority if budgets limited

He's a passage from EST CE101 2006 .
http://www.energysavingtrust.org.uk/corporate/Global-Data/Publications/Domestic-energy-efficiency-primer-CE101-GPG171

A major source of energy loss is through windows and doors.
Energy-efficient windows, when correctly selected and installed, will help to minimise the heating costs and
will also increase comfort.
Although savings from installing new windows are not as high as other measures (e.g. cavity wall insulation), it is important to recognise that windows are replaced very infrequently so another opportunity to install high-performance glazing may not arise for a number of years.
Secondary glazing is a good option where thermal performance needs to be improved and the existing
character of the dwelling needs to be maintained.
Draught-stripping of existing badly fitting windows and doors is inexpensive and simple to install. It can
greatly improve comfort as well as reducing heat loss.
2.6.1 New windows
When replacing windows, high-performance units should be specified as follows:
Ideally, they should achieve a BFRC rating in band C. The ‘energy saving recommended’ logo will also
be present on windows in band C and above
Payback by specifying a BFRC rated C window, as opposed to E, is usually achieved within 5 to 6 years.
Replacement windows should always be fitted by a reputable installer and sealed around the
frame (to reduce draughts and air leakage at the wall-to-frame junction). In England, a FENSA
registered installer should be used (please see ‘Relevant organisations and websites’).
Replacement of windows in historically sensitive buildings should only be undertaken after consultation
with the local authority’s building conservation officer.

From CE83 .Energy-efficient refurbishment of existing housing
http://www.energysavingtrust.org.uk/business/Global-Data/Publications/Energy-efficient-refurbishment-of-existing-housing-CE83-GPG155
The greatest impact comes through the reduction of
infiltration heat losses from minimising draughts

Whilst not untrue, that quote gives an excessively positive view of the value of replacing windows. it could have been written by the GGF!

The trouble is it all depends on individual cases, but assuming a completely unimproved house, windows are the last thing to replace, if the existing ones are sound. Obviously, if the windows are to be replaced it makes sense to use high performance windows but all things being equal the heat loss through draft proofed single glazed windows isn't that high as a proportion of the total heat losses in a building.

The old rules of thumb of cost effectiveness still apply.

Cheapest and highest return - draught proof windows and doors.

Ensure that there is at least 200mm of loft insulation with no gaps.

If you have a hot water cylinder give it a good thick gapless jacket.

If you have a cavity wall without insulation get it insulated.

With only £2000 to spend windows would definitely be last on the list.

These DIY calculations from a USA, 1970s refurb shows a 1.6% ROI and 72 year payback for upgrading windows

http://environmentalchemistry.com/gogreen/ecorenovation/70shouse/revisedreturnoninvestment.html

I noticed a HUGE difference when out old metal single glazed units were replaced with cheep upvc one (not very green I know, but its a rented house and not my call)

The old windows were very drafty and I think from a subjective point of view the draftyness (or lack of) was the biggest improvement.

It ment also that we could tolerate the CH stat set 2 degrees colder. We used to need it on 21*c to keep the house comfortable, and we can now tolerate set to 19 to get a similar level of comfort.

As such we have saved not only through improved window U value, but reduced draft and as such being able to tolerate a lower air temp!

Timber

This sounds like 'how long is a piece of string' (answer: always an inch too short)

You have to work out the areas of the windows and the areas of the wall/floors/ceilings/roof, their corresponding U-Values and do the sums. That is all there is too it, apart from knowing your local climate regime, and knowing how a spreadsheet works (though I often find paper, pencil and calculator is useful).

If they are timber windows, have a look at just changing the glazing. I managed to fit new 4-16-4 units into my existing frames (also painted and repaired them) for about £600 for 13 windows each approximately 48" by 24") second best investment I have done, the first being £12 on V-Profile door draught excluder.

Nick