Well, you can fence your old deck, for one, to finance the upgrade - fencing most gear nets (25 + 5*Net Hits)% of the item's cost, which can be a pretty significant chunk, especially if you've got a good Face to do this for you. Those payouts do seem a bit low to me, though... Saving for, say, Delta Wired Reflexes 3 or Beta Synaptic Boosters 3 would take freaking forever (the former costs about what the Shiawase Cyber-5 does), and if you can sell what you're replacing there at all it's for a pretty diminished value. Trying to advance a Rigger on that would get pretty seriously rough, too.
The important point, though, is actually to balance the nuyen rate against the Karma rate - the two are asymmetrically required by different archetypes for their advancement, so if things get out of whack advancement gets unbalanced.