The amount of savings really varies a lot. Typically, FT members use eVIPs to upgrade international flights, and consider using them to upgrade domestic flights a waste unless they are about to expire. But international J varies a lot by destination, and often discounted business fares are available. I have seen examples posted here, for example, of people using eVIPs to upgrade a $400 coach seat each way from JFK-Zurich to a business seat that would have cost $2,000 each way, so that flyer would have saved only $1,600 per eVIP. But IMO it doesn't really matter that he saved only $1,600, if he wouldn't have bought the business class seat for $2,000. He also wouldn't have bought it at $4,000, so its price, and the amount of savings, is not relevant.

I guess what I am saying is that if you regularly fly paid J, then it would be smart to pick the flights where the difference is greatest, and buy coach tickets for those flights and upgrade them to business class with eVIPs. If you don't, then I wouldn't pay any attention to the cost difference, and I certainly wouldn't pick destinations or flights where the cost difference is greatest. I would go where I wanted to go, and enjoy the ride in business class.

Enjoy your flight in business class.