He has a very large family, around 80 just in uncles,aunts and cousins?

We are paying for our wedding ouselves and live in S.F CA so it is very pricey. His whole family expects to come which will push our guest list to aprox. 150 + people. They expect a nice reception with alcohol etc but we can't afford this. They all are expecting to be invited and will be very disappointed if we elope. He is Irish Catholic so the traditional wedding thing is very important.All his friends brothers abd sisters have had very large affairs. What should we do ? I want to elope but I don't want to hurt my family especially my mom since I am her only daughter.