Notwithstanding the various opinions and suggestions put forward in regard to this particular situation, please don't make the mistake of thinking "only 15%" (or whatever, depending on no. of kids) is automatically "affordable by NRPs. Obviously "affordable" - or for that matter "enough" - is a subjective term, but bear in mind that individual families can live in an infinite variety of circumstances and the CSA's "one size fits all" formula, which is very black and white, does NOT take into account the grey areas in which most of us live.
Many NRPs (usually men) find themselves having to start again from scratch post-divorce .... e.g. high loan to value mortgages over a shorter term as they're older. This happens when the ex has recieved the bulk of the former marital assets, and if, that means, as in my DP's case (and other men I know) that the ex wife herself ends up with little or even no mortgage, she - and the children - will effectively benefit from NOT paying several 100 pounds each month in housing costs. Many NRPs, through absolutely no fault of their own also have to pay 100s of pounds every month if they want to see their children at all, because the ex has moved away - then subsequently refuses to help facilitate contact by meeting half way, sharing fuel costs etc. Some NRPs are also servicing marital debt which was made their sole responsibility as part of the divorce process.
So ..... the neat CSA percentage formula does NOT always "ensure" that NRPs have "enough" to live on ..... because it doesn't recognise the redistribution of assets and so on, nor what most responsible NRPs would argue is the essential expense of travelling to maintain a relationship with their kids.
I know of course that logistically there probably isn't any way the CSA could ever operate differently and come up with a magic solution whereby everybody was treated fairly and individually at the same time. Of course it couldn't, it couldn't possibly be administered. They have enough trouble applying a simple formula as it is. However, CSA aren't as cut and dry as some people seem to think they are and as Incognito pointed out, the NRPs child-related expenditure doesn't end with the CSA (assuming they are trying to maintain a normal relationship with their children). My DP has significant extra expense due to his ex refusing to meet him at any point along the route at all, and has had to pay this for years. We've also had to waste money completely unnecessarily for many years by providing a complete wardrobe here for the children because the ex refused to send them with any clothes .... money which could, arguably, have been used to benefit the children in more useful ways, such as extra activities. I'm afraid that I do regard our particular CSA payment as both unfair and unaffordable because of our particular family circumstances - would never suggest that the CSA formula is unfair across the board. That remark may get me shot down in flames but DP's ex - and as a result, his older kids - have benefitted hugely from the way finances were sorted when they divorced - and also from many years' worth of inflated (by several extra 100s pounds per month over CSA rates) informal payments which set them up very comfortably and provides them with a decent standard of living. In our case, it was DP who eventually approached the CSA because his ex refused to discuss the situation with him and we were sinking deeper and deeper into debt trying to sustain payments completely out of kilter with our income (had been informally agreed prior to DP being made redundant). Certainly, when you add up the CSA payment, the travel costs, the cost of feeding and entertainment - and that doesn't include us having to have an extra room for DP's kids - far far more is allocated to each of DP's older children than we could ever hope to spend on our own daughter .... what we (i.e. two working adults) spend jointly on our DD every month is probably about 40% (roughly) compared to what one parent (DP) spends on each of the other two. See .... it's a complete grey area and whilst I know this kind of thing can swing the other way and of course there are many PWCs left high and dry by irresponsible and uncaring NRPs who've pissed off without a backward glance, I just wanted to show how paying "only 15%" (or 20%, 25% as applicable) doesn't automatically mean the NRP is left rolling in it.
Apologies for going off on a tangent there ..... back to the thread. I'm of the opinion as Redhen says that you shouldn't spend someone else's money for them. The ex should have discussed this before going ahead and whilst it's great - for SD - that you're going to contribute this time you do need to make it clear to her that she may get a very different answer if she does similar next time. Just as PWCs need regular and reliable maintenance payments so they can budget, believe it or not NRPs also need to budget and can't be expected to find additional expenses just like that without any warning. By the way in case anyone thinks by what I've said that I'm hard and unfeeling, I was actually a single parent myself for 9 years before meeting DP so I know that where all parties can be mature and fair about it, it is possible to thrash out a fair and honest maintenance system without involving the CSA. I rarely used to ask my ex for extra money but on the occasions where something unexpected cropped up, or where expenses had increased dramatically for some reason, I'd always speak to him about it before committing money I didn't have.