Almost. The enchantment costs 8k. The 50 masterwork arrows would cost 302.5 gp (1 gp per 20 arrows, + 6 gp per arrow for masterwork; 50 masterwork arrows cost: Arrows (50/20) * 1 gp + Masterwork 6 gp * 50) for a total cost of 8,302.5 gp for 50 +1 flaming arrows, If I am remembering correctly. It may just be 350 gp for 50 masterwork arrows, though, as I could be wrong.
However, while at first glance it seems to make more sense simply to enchant the bow, consider:
You can split 50 arrows up between five people, and have everyone firing +1 Flaming arrows for 10 rounds. You can't do that with one bow. Sure, the arrows run out, while the bow doesn't (well... it runs out of mundane arrows, but those are cheap), but it's a trade-off to consider.
You can use +1 Flaming arrows with a +5 Bow, and shoot like you had +5 Flaming arrows, at a cost of 50,375 (Bow) + 8,302.5 (arrows) = 58,677.5 while a +5 Flaming bow would cost you 72,375 gp. Again, you will run out of your magic arrows (eventually), while investing in a bow will likely give a better long-term return on your investment (unless your DM has a habit of Sundering or Disjunctioning your stuff)
In the extreme, you can use a +5 Holy Speed Bow (+10 equivalent, 200,375 gp) with +1 Brillian Energy arrows (+5 equivalent; 50,302.5 gp) to get +5 Holy Speed Brilliant Energy hits (+14 equivalent) with non-epic rules.
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