Hello - I went to university in the US and used to work as an admissions consultant (about a decade ago, so the info is a little out of date, but mostly still true).
US college decisions are made before A levels would even be taken, so what matters is grades (marks) up until that point, plus the results from standardised tests - either the SAT (not UK SATs - a different test!) or the ACT. Different colleges require different tests so it is important to research the colleges to which you want to apply first, and figure out their entry requirements. Your DC will need to study for these tests - though they are supposedly not something you have to prepare for, the reality is that most American kids do some form of test preparation, and the tests are in a format and ask a style of question that will be very unfamiliar to someone from the UK school system.
Each college has its own application, though many colleges use the Common Application (though they may well have their own supplementary forms). The process is much more complicated than applying to university in the UK, and applications often have multiple additional requirements. If they are applying to a very competitive college, they may need to write up to five additional essays, plus get multiple letters of recommendation etc. You need to set aside plenty of time for the college application process - it will take several months rather than several hours. You will also need to pay an application fee to every university - these can vary from $50 to $100.
In terms of making the decision to go: as PP have said, it is very expensive (far more so than any UK university) unless you get a full scholarship / significant financial aid. There are two main ways of getting financial help.
The first is via sports scholarships - if DC is a very competitive sports player (nationally ranked) then they may be able to get a sports scholarship at universities that particularly value that sport. This can be a great option for someone who is very serious about their sport. However, it is worth noting that these scholarships are tied to playing that sport for the university - if DC decides to stop playing, the university may stop paying.
The second way to get financial help is by going to a "need blind" college - usually, the most competitive colleges (e.g. the Ivy League, Stanford etc). When you apply to these colleges, they don't look at your ability to pay. If you get offered a place, they will work with you to make it affordable through a combination of scholarships and small-ish student loans (depending on the college). There may also be a work-study element in which you have to have an on-campus job to cover certain expenses - every college is different.
If DC is unlikely to qualify for the types of help listed above, then you should be prepared to pay anywhere from $40,000 per year to $90,000 per year, depending on the college. This would include all on-campus living expenses, but remember that US college degrees take four years, so you would be looking at a minimum of $160,000 for four years (and this would only be at the very cheapest and thus no-terribly-good colleges).
If your DC has exceptional talent either in sport or in academics, then it is definitely worth looking into. I went to an Ivy League university and it was an amazing experience. However, the top colleges are extremely competitive - DC would need to be someone who had been recognised beyond their school for their academic ability (by winning academic competitions or some sort of national ranking, for example), or have some other truly exceptional talent.
If your DC is a good but not exceptional student, and you have any sort of financial restrictions, then they would probably be better off at a UK university. The quality of education that they would receive at a mid-ranked university in the US would be no better than what they would receive at a mid-ranked university in the UK, but it would be significantly less expensive and involve a lot less time and effort to apply.