That's podcast doesnt say too much different that what I'm saying.
They state that the RDA calculations give the vast majority of people enough (bell curve) so some will be eating too much.
They work on 40g being "enough". (But dont specify enough for whom. They acknowledge need more if you are a child, pregnant, breastfeeding, exercise regularly etc).
To get enough protein you still need 7 portions of lentils a day. That's 210g of lentils at 50p and 539 cals
Its still 2.5 tins of chickpeas for 40g of protein for 690cals at £1.92
Only 160g of chicken (mid range chicken tesco) at £1.15 and 176cals
Etc.
To put that into context, you need to be eating roughly two portions of lentils at every meal. After that your not necessarily saving money if your eating a variety of plant based sources.
Also note that as your podcast suggests, this won't be enough protein for everyone.
To bring it into (bitter experience😁) real world context of a teen on free school meals who only eats a pizza bread/sausage roll/ portion of chips/sandwich at lunch (because no time/queues too big) and exercises/walks to school 2/3 times a week. Realistically you might have up to half the calorie count for the day down and very little protein. If you want to try and balance off their diet at home you're going to need to work hard at it to make a lentil based diet work.
It takes waaay less thought to do a random choice of meat, 3 root vegetables mash plus brocolli and peas or a chilli or bolognese.
I'm absolutely not saying it can't be done but but to make it work in a real life context and save money and get enough protein you've absolutely got to be committed. Over your entire diet.
Most people will just replace portion to portion which does not work.
There's a lot to be said for reducing your meat consumption and eating from some veg based protein sources as opposed to straight replacement.
(I am absolutely not a fan of protein bars and shakes etc. Very proud of DD when she pointed out to her gym bod mate he could get the same protein from drinking milk and save money!!)