A more affordable way to get a Saga seat (but not Saga service) will be to book premium economy. If Saga is not sold out, the last row(s) of the business seats will be available to the premium economy passengers.
On my recent trip, 3 out of the 4 legs were available and per-assigned.
I could not justify the cost for the Saga class, but could for the premium economy one. Considering you can pre-order your BOB (which is free and unlimited - you can get as much as you want), it was a decent deal.