In among a raft of new infrastructure spending announced by the UK government in the wake of last week's spending review, it was revealed that the cost estimates for the HS2 high-speed train line had been revised significantly upward. According to the new projections, HS2 will be completed in 2033 at a total cost of £42.6bn for construction and £7.5bn for trains – a total of just over £50bn.

What is immediately striking about this figure is that it's about the same as estimates of how much it will cost to develop nuclear fusion to the point at which it could supply affordable electricity to the grid.

Fusion power has the potential to revolutionise the entire world's energy production. It could dramatically reduce the world's carbon emissions (a fusion reactor emits no carbon dioxide), provide energy independence to any nation with access to a coastline (since there is millions of years' worth of fusion fuel in the world's oceans), and do all this with no danger of meltdown or long-lived radioactive waste.

Alternatively, we could use our £50bn to shave 35 minutes off the journey time between London and Birmingham.

Whatever you think of the arguments for and against HS2, estimating its economic benefits is a fraught business because it's so difficult to predict how it will fit into the UK's broader transport infrastructure by the time it has been completed. By contrast, the economic, environmental and social ramifications of almost limitless clean energy have the potential to far outweigh those of a new stretch of high-speed train line connecting a handful of cities on a small island.

But can we really have fusion for a few tens of billions of pounds? Obviously, no one knows for sure. If we knew exactly what remains to be done, we wouldn't need to spend all that money on research to find out. However, there are reasons to be optimistic: we've got fusion working, albeit fleetingly, at the JET reactor in Oxfordshire, and consequently we have a good idea of what issues need to be overcome to progress from experimental devices to working power plants.

"Twenty-five years ago we didn't even know if we'd be able to make fusion work," Prof Steve Cowley, director of the Culham Centre for Fusion Energy where the JET reactor is based, told Popular Mechanics recently. "Now, the only question is whether we'll be able to make it affordable."

There are doubts about the accuracy of estimates for HS2. Commentators are generally cynical about enormous infrastructure projects, since they so consistently overrun in both budget and timescale.

Guesstimates about the cost of developing fusion are probably just as accurate (or inaccurate) as those for HS2. Unlike HS2, though, there's a possibility nuclear fusion could come in dramatically under-budget. The EU's roadmap to fusion backs the brute force route – because experiments in a large reactor make the physics somewhat simpler – but a sprinkling of support for smaller-scale, more esoteric routes to fusion power could lead to a breakthrough on a far shorter timescale.

Fusion experiments, like all scientific research, could lead to spin-off technologies. The work also supplies highly trained personnel for the wider workforce, and is an inspirational technological and scientific goal on the scale of the moon landings or the Large Hadron Collider.

Did I mention that we could use our £50bn to shave 35 minutes off the journey time between London and Birmingham?

The point of all this isn't to discredit HS2. It is to illustrate that governments have these amounts of money, which has the potential to change the world for the better, in their back pockets. Nor is the point to promote fusion to the exclusion of other energy research: it is certainly not unique in its potential cost-effectiveness. I could just as easily make a compelling case for thorium-fuelled nuclear reactors – which are safer and produce less radioactive waste than conventional fission reactors – or investing more in renewables research.

Biomedical research is another obvious avenue with enormous potential for improving the quality of our lives, for example by improving survival rates in cancer. A back-of-the-envelope calculation suggests that, for the cost of HS2, additional funding for cancer could add around half a year of life expectancy to everyone on the planet.

Investment in scientific research pales in comparison to the size of the problems science is trying to solve, and the cash for huge infrastructure projects that governments seem quite capable of finding.

Andrew Steele is a bioinformatician at King's College London. He recently launched Scienceogram UK, a campaign that compares the benefits of increased investment in science with big-ticket projects. He tweets as @statto