I would ask is your heart really set on a beach wedding or a beach wedding in St Lucia. Because maybe you could change it to an island a little less expensive. Also are you expecting your parents to stay for an extended amt of time bc if so maybe they could stay for only one or two nights. Maybe they could stay at a hotel that's cheaper
I think there are many options out there you guys just need to sit down and discuss this rationally. Or like someone stated earlier you have to figure out ultimately what's more important to you, having dad walk you down the aisle or getting married on a beach (in St Lucia).