Instead of just setting price to 0, here are some examples for setting the price to 1/10th it's current price and then setting sellrate to 10, which should mean the vendors pay 1/10th what they did before, but sell them for the same rate as before.
I haven't tested the UPDATE query below, but it is based on the SELECT query, which seems to work fine. Feel free to mess around with them to have them do what you want. Probably want to test them on a non-production database before you update your real one though.
Code:
SELECT tradeskill_recipe_entries.item_id, tradeskill_recipe_entries.successcount, items.id, items.sellrate, items.price FROM tradeskill_recipe_entries, items WHERE tradeskill_recipe_entries.successcount > 0 AND tradeskill_recipe_entries.item_id = items.id AND items.sellrate < 10;
Code:
UPDATE tradeskill_recipe_entries, items SET items.sellrate = 10, items.price = (items.price / 10) WHERE tradeskill_recipe_entries.successcount > 0 AND tradeskill_recipe_entries.item_id = items.id AND items.sellrate < 10;