Is the price a bit high for an R10, even in great shape?
You can say that again. When appraising a Darne R model, START WITH THE TRIGGERS! There are some that simply can't be made better, and if you find a good one that has decent triggers, it is worth more money. While a great fuss is typically made about chambers, a Darne will always have plenty of material in that area of it's barrels, and lengthening them on a hunting implement like an R10 isn't a huge deal. I run the Winchester low noise 2 3/4" loads in my 65mm chamber Halifax with no problems, another option to consider.
Or, you could just buy the right ammunition. Sé vous plais.
Nice as it is, I'd hate to be into that R10 for more than $1500-$1800 or so. Assuming good triggers.