Since your famiy is the largest, have a sit-down with your parents and grandparents. Tell them you and your beau have budgeted only for 150. Tell them they need to help you cut people from the list. If they don't want to, then they should pay for the excess guests. I don't think you should skimp on food on account of all these people. Perhaps your closest family who were going to give you cash as a wedding gift can pre-pay so you can pay for plates (I only recommend this as a last resort, not really appropriate).
If all else fails, you and your fiance should make the decisions who whom to cut, cut them and be done with it. Whomever isn't happy will have to live with it. It's your day, your budget and you deserve happiness without all of this stress. Everyone is an adult and should understand. You have nothing to feel bad or guilty about, just make firm decisions, stick with your plan and have a great and memorable wedding. Best! Lise'