MLCS (mlcswoodworking.com) has decent-quality roundover bits from 1/8"
to 1/2" radius for $13-$17. Or you could go even cheaper and get some
no-name chinese bits, which would probably be fine for the edge of a
deck. At that price, you could burn through several bits before it
made any significant difference in the price of the deck. If you were
happy with a 1/4" radius roundover, you could probably do it all in one
pass. Keep in mind when routing end-grain it's likely to chip out at
the edge of each board. Sounds like a lot of work to me, but if you
really want your deck to look like all your neighbors' decks, it is a
possibility. What about running a circular saw along the whole edge,
with the sole plate set on a bevel, and taking off a 45 degree angle
bevel about half the thickness of the board? Wouldn't be round, but it
would be "eased" so it's not a sharp edge.