Considerations are:-
Resolution - have your laptops, PC's out stripped the resolution of the projector? Latest laptops have different screen ratios and resolutions which will put the projected image under strain. Think of what your pupils are having to put up with, are they getting eye strain?
Image - if you have any discolouration of the image (blue blobs, Nicotine stain, greenish hues) then your additional concern is dust and dust burn, if it's blue blobs (best seen if you move zoom & focus to one full stop) then you've got dust on the internals, which if left untreated will over heat the unit and lead to the next issue of dust burn where lens surfaces can become pitted and damaged through simply having hot dust on the surface.
Apart from the green perspective in the current economic climate the trend is to make do with what you've got but with that comes other issues, if you want to get the most out of your existing equipment you need to put a proper maintenance program into place. Just like your car you'll get more mileage out of it if you service the units regularly. Even if it's new you and still under warranty, you should maintain on a regular basis, (again like your car - warranties are void if you don't service).
So it's not just the cost of the bulb it's the other stuff that goes with it on the up side, if you do continue with existing stock, you don't have the disruption and cost of projector refits involving new bracketry, and cabling.
Checkout AV preventive maintenance companies, there's one I know in Cwmbran that also has a lab where they can rescue sick projectors.