It obviously depends on the product and contents of the upsell, but in general I would always aim for a higher priced OTO that both complements the original product, and offers much more "advanced" material with a high perceived value.

Not everyone will take it of course, but that does allow you to offer a lower priced downsell also, by taking out some of the content, bonuses etc of the original OTO.

I've seen some people go crazy on this and offer 4-5 downsells, but with that you run the risk of annoying your buyers.

As Rob said, just make sure to overdeliver, no matter what you do.

Don't forget you can still offer the upgrade later on (at a slightly higher price) and if your front end product has impressed people, then they may be more receptive to upgrading as they will know the kind of quality they can expect from you.

Hope this helps,

Richard