The problem in a job like this is going to be labor and, also, other parts. The folks out of RPI Engineering in the UK (who just go goofy over these V8's) will tell you that your timing chain will be experiencing stretch at this point as well. So, to do a lifters/cam follower job right - they will sell you a package at around $700.00 (USD) that will include an upgraded cam (i.e., good for low-range to mid-range revs performance and improved fuel economy), lifters, performance timing chain and both gears, shims to bring your lifters to correct height once on the cam lobe (a critical consideration that should not be overlooked), and all the gaskets necessary for the job. I can't remember if they include push-rods in that package - but they are not expensive as a rule anway.
The reason you may (emphasis on that word 'may') need all these parts is that even at 68 - 70 k those parts from the cam through to your rockers will have worn in sympathy with each other - and it can become a real crap shoot to just replace lifters without doing the cam and if you are going to do the cam - at that mileage - why not do the timing chain and gears, etc? Or so the logic goes.
A really smart mechanic who is very familiar with these engines may know just how to tickle things so that you can get away with only the lifters being replaced but most of those engine specialists (when it comes to Rover v8's) are on the other side of the Atlantic in the UK (ironic is it not - considering this particular engine's original design comes from GM's lowly Buick 215 CID back in the USA).
What labor would cost on top of those parts is something I would not hazard a guess on for your area; but, from my neck of the woods it would probably run somewhere between $700 to $1000 Canadian Dollars.
So, check this out carefully and if you are getting reasonably good performance and can still quiet that tick with additives - you might want to wait a while.
Wolf
