Is it okay renegotiate salary after agreeing to a figure in the interview?

I am a final year PhD researcher in Western Europe. Recently, I interviewed with a company that I really like; the role is interesting as well. In my first interview, the hiring manager asked me what my salary expectations were and I quoted a figure which was higher than my current salary (stipend). He told me that it's too steep.

During the final interview, the team leader asked me the same question. I quoted the same figure as I was scared that it was already too steep. The manager agreed and sent me an offer letter the next day.

However, now that I check the living expenses in the city where I will have to move to, I realise that it is much more expensive that my current city. In fact, I could end up saving less than what I do now.

Is it possible, at this stage, to renegotiate the salary? How do I approach this situation without jeopardizing my chances at the company?

Thank you for your help.

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