How much does the price of a new bicycle drop a year after buying it? I'm planning to buy a bicycle "on a budget" (I'm sure yet if I should buy a new or an old one). But since I'm going to use for just a year, I'm wondering how much I'm going to sell it for next year (how much I'm going to lose in it purchase price)?

I don't know if what they say about cars (that you lose 20% of a new car's value after owning it for a year according to Car Depreciation: How Much Have You Lost?) applies also to bicycles.
I would use bicycle blue book as a way to figure out the change in value of the bike.  They have a system to estimate the resale value of used bikes.  Your bikes condition at the end of the year will matter so keep it clean and lubed.