I think you may be confusing two separate quests. Age doesn't guarantee quality. In my experience, some of the best malts are in the 16-24 age range, although some younger and some older whiskies are also very good. Older malts tend to become rather dry, woody and one dimensional.
If you are wedded to the idea of getting something over 30 years old, then Ben Nevis 32yo comes in at around £100 at the Whisky Exchange (I've never tried it, but the 26yos have been exceptional), and Tullibardine 1973 (Tullibardine website) comes in at £90 and got an excellent review in JM's 2006 Whisky Bible.
If you are interested chiefly in age, you can get some Strathisla whiskies bottled by Gordon and Macphail at the Royal Mile Whiskies website - they have a £40yo for under £95 - but again, I have never tried it.
I wonder though. If your grandfather really likes whisky, he would probably appreciate a carefully chosen bottle regardless of age. If he doesn't know much about whisky and you want to impress him with age, then you might as well do it cheaply.