I was in this situation and decided to reject the offer. It would have been my ideal PhD as I loved the topic but knew I wasn't prepared to self-fund as it would have just been too stressful constantly worrying about time and money. The PhD I'm doing leaves me cold but it provides an income and, for me, that's a great relief. I suppose it depends on just how much you love the topic, if you can afford to self fund or can find a way to self fund and what your end goals are and how helpful this particular PhD would be for you. Completely your decision but I'd be inclined to look at other PhD's with funding.