I've the same conundrum, was quoted 20k net of grant last year to do my (detached) house. My gas bills cost roughly 1000 per year so even if it saved me 33% as the installers usually claim, it'd take me 60 years to make that back. OK gas prices and carbon taxes will likely go up in the future but the ROI would still be extremely long. Hopefully they'll be better grants/incentives in the next few years to make this more viable.
If I needed to get the house re-plastered anyway though I think I'd probably go ahead and do it.