I think the lack of expertise also makes it more expensive. China has built a huge amount of high speed railway over the past decade (I think they basically layed track down across the entire country). They have probably refined the process significantly since the first stretch they layed down, which would help to drive cost down as well as build time.
It's one of the reasons I hope they decided to just lay down high speed track all across the country, rather than stop and come back in about 10-20 years to do another small chunk. Build up on what they learnt from HS2, if we get good enough maybe we could export that skill.
It's more that China is a huge country and with very low labour and land costs (I would also suspect that China wouldn't admit to cost overruns).
We've got most of the expertise we need to do railways, a very high percentage of the cost and time is simply the planning and approval processes, in China if the government says "we want to run 200km of track in a line from A to B"" they can do it without most of the delays we have, and paying a fraction of what we pay to any private land owners. China was forcing entire villages to relocate for some of it's projects and often people were not too unhappy as they may have been getting moved from very basic, very old housing to something that was much more modern/easier to live in just a few km from their old houses.
Yes spending years building does help with the experience, but the same would happen here, but we would still have the same issue we have now, in that any route you wish to run track through is almost certainly going to require moving a lot of people out of their properties, so you end up spending years trying to find the route that will work technically, and with the least number of displaced people, then spend years more fighting through planning permissions and appeals to get approval to do it, all of which means that even before you've moved the first bit of soil for construction you've potentially spent a good fraction of what it costs in some countries for an entire project, just in paperwork.
And whilst we're dealing with paperwork and appeals the original cost based on predictions of pricing of materials is now getting more and more out of date, and simple things like inflation are now eating into what was the contingency fund. IIRC one of the reasons for the HS2 cost overruns is that the price of steel went up far more than anyone predicted early on (it also affected pretty much every construction project planned in advance, but obviously HS2 uses a lot of steel).
It's one of the things that makes comparing costs of infrastructure across countries so hard, as in some countries the land cost is next to nothing (already owned by either the organisation doing the work, or by the government), in others you might be looking at 10's of millions pounds per km of track just for the land and compensation for lost property values.
