Interestingly, the hours life on Bulldogs not fitted with a fatigue meter must be based on worst case usage, as typical ex-RAF ones will have 8,000 to 9,000 hours, at 90 to 95% fatigue life. Straight and level flight adds nothing to the fatigue reading (but each landing is logged and has a tiny effect on FI), so with gentle flying, airframe life should not be an issue.
The current estimate on the spar mod cost is a similar order of magnitude to an engine replacement - TBO 1,800 hours. We all mentally or physically allocate an 'engine fund' - say £10 per hour, so another £10 per hour towards the spar mod isn't a huge deal... for a private owner for whom all this is some way off in the future. As a business proposition, it's clearly a different matter, as time to mod will be much shorter, down-time while it's being done means no income, etc.
Once done, the mod extends the FI from 114 to 200, so almost doubling the airframe life. I hope Bulldog owners can pull together and take advantage of economy of scale when it comes to commissioning a run of bolts, etc. That way the cost will be reduced for all of us.