U can get a used Dj with very good gear for under 500 eu. But for custom build make that twice. I give you my example , I payed 1600 eu for my bike, It was builded allready , I rided it one year stock , did just a tiny upgrades. But for next year i have plans for upgrades that will cost 2k eu
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and that is only front fork, rear schock and maybe new brakes..
E: And get your facts right pls , you wont save nothing with custom build. You only save if u buy fully build bike...