^ No, I don't think the Middletons are panicked by the cost of Pippa's wedding. It's low key by the standards of many of Pippa's friends. George Percy's sister Missy for example had her nuptials at the family home Alnwick Castle and the church nearby. There were multiple groomsmen, guests stayed overnight at the castle and the villagers turned out in droves. When the eldest van Cutsem son (son of Prince Charles's friends) married a Duke's daughter it was in Exeter Cathedral and the reception was at the Duke's nearby country home. That's an aristocratic wedding.
By contrast Pippa will be marrying in the small church nearest her home, we don't know how many attendants she will have, but it's unlikely to be a dozen. The reception is likely to be a marquee in the gardens of the family home.
Because of who Pippa's sister and brother in law are, there is likely to be media interest, so some expense will be involved in keeping paps etc away.
Apart from that, Mrs Middleton and Pippa would no doubt have been offered excellent deals ref the catering, cake, cars, gown etc. I wouldn't be surprised to discover that a contract has been signed with a magazine like Hello to cover the reception. With the payment from that they would be able to have a lavish wedding and still have money left over, as Peter and Autumn Phillips did.
So over all no, even if there is no magazine deal I don't see the Middletons struggling financially with that little wedding.