I have recently received an offer from a group which pursues topics I find very motivating and whose equipment is above average for my field.
The salary offered is 50% of the German TV-L E13 salary, which I think is acceptable by academic standards. However, all other offers which I have received are 65% of TV-L E13, though all of these other groups pursue topics which to me are slightly less interesting. To give you a bit of background, 50% has been the standard for a long time, but since a couple of years ago most groups in my field are deciding to award their PhD employees 65%.
I am very motivated by my particular interests, and had the salary been equal for all offers I would have taken this one without question. Sadly, the situation being as it is, I am quite split. Additionally, the professor made it quite clear that I would also have to get involved in teaching (not all other groups were as forward about this) - which I do not mind in the slightest, but which I think would give cause for extra consideration when calculating a salary. Regarding the professor's interest, he seemed very keen to have me on board, but also very confident I would take his offer.
I would like to ask whether he could accommodate for 65%. But, since I am yet to decide, I would not like to lose the offer or put great strain on our future relationship if he refused.
The way I see it, academic culture treats monetary interest with a certain disdain, and I would like to ask you how I could best formulate my request.
FEEDBACK: I have followed the advice offered here, and stated in an email to my prospective PI that I had received better paid offers, and that I would ask him if he could increase the pay, though I am aware of how difficult this could be for him. He took some time to reply (which initially gave me cause to worry), but apparently now he managed to arrange for 65%. I would encourage any of you to follow the advice presented here; and do try to negotiate as long as you do it very respectfully!