Reconsidering Low Balled Start up offer

I declined an analyst role from a startup because the offer was a lot lower than what i make in my current job. 18k lower... i think this startup will give me the opportunity to be an analyst and then also learn how to be an engineer but the pay just made me feel like i wasnt being paid what i was worth im currently not doing any analyzing of data or engineering right now but ive applied to over quite a few jobs (300+) and havent received anything offer so i feel like im not worth anything should i reconsider this position if they would let me accept the offer after ive already declined? i told them i was going to look for opportunities at my current job. also how bad would it be if i get this position and then leave after a few months because i was offered something better? (please answer in comments)