I agree with Dan Anderson's idea, don't say anything and just promise to rectify the situation next year.
In terms of dealing with it next year, I've been running a Secret Santa event with my friends for what will be the 4th year running now.
The way we deal with people not being happy with gifts is in advance, everything is done via a web app I made so this makes things a little easier for us.
This includes a rules page, stating exactly what is an isn't allowed and even picks lots for us with an opt-in/out system. This helps to separate the system with any one person, as we've all read and agreed to the rules and a computer chose who got who.
In terms of the money amount, one of the rules is this:
"Total of gift(s) must be within £4-£5. If you really want to spend more nobody will stop you. But expect to get a gift of this rough value."
This has been set low mainly so that people who are poorer can still participate without someone getting annoyed because "they invested more", each person knows exactly what to expect out of the exchange at minimum. If someone were to go massively under (say, £1) words would be had and they'd possibly be removed from the system.
There have been incidents where someone wasn't happy with their gift or we got too much of X, but our system allows easy modifications to a rules page everyone could read. One year we got all mugs, so they've been banned for now. This might sound a bit dictator-y, but it means that the spirit of 'think about your gifts' is kept alive through 'curating' of the whole thing.
I'm not suggesting you code your own web app for Secret Santa (I can drop you on my system next year if you message me), my idea was a bit elaborate if I'm being honest. But you need to have a list of rules somewhere easy to access (a g-doc everyone has the link to, for example) and you need to make sure people have read the rules before participating and understand their minimum obligations.
If anyone does all this and still breaks the rules, deal with that depending upon severity. For an issue in budgeting, having a quick word so they understand what they need to do next year is best. If they do it again, kick them out.
I'd also consider lowering your minimum budget or giving some leeway on the amount (I personally recommend £5 for Secret Santa's), you don't want your poorest users to be left out in fun events. The point is just to buy someone a token of appreciation rather than an amazing gift in my opinion.