Thanks, ladies.
Being well endowed is not a situation I've ever faced (nor am likely to), but I was largely introduced to lingerie making by a friend who has a narrow back and large cup size and subsequently does struggle to get good fit in her bras. To get this in RTW, she estimates it costs her around £75 for a bra.
That said, the ones I've made are far more comfortable than RTW and, of course, the variety is infinite, which I really enjoy. I'm losing weight at the moment and when I get towards my target weight, I know that I will go down to an A cup. This brings its own fitting problems in RTW, not to mention low availability of nice bras in an A cup that aren't designed for teenagers! The distribution and placement of my 50 year old boobs is very different to those of an 18 year old, even if there isn't much of them.
I've been watching a Craftsy class by Beverly Johnson on designer bras recently and have really enjoyed it. Her basic bra making classes are very good as well. I reckon that if you have to spend upwards of £50 on a single bra, the time and effort of learning to make your own is a worthwhile investment if you have the means to do it.
I found the thought of sewing a bra to be very intimidating when I first tried it - very alien territory for me and completely out of my comfort zone but it was worth perservering.