Tell the truth, that the trip would be unaffordable but don't tell them how much the cost would be, even if asked, as this might come across as an expectation that they might contribute, which could be embarrassing. Send a lovely card and maybe a well chosen modest gift, like flowers, a pot plant or plant for the garden, nice chocs or a bottle of fizz, as this can be done by post and perhaps a phone call a few days on advance to wish the a happy celebration.. None of these gifts would be house clutterers and are normally considered thoughtful
gifts, and enjoyed by nearly all recipients.