You can buy Usborne books from discount book stores, outlets and the book people too, and then they will be similarly priced to any other books you buy through those avenues.
You can also buy them through e.g. Waterstones, which means they will be full price but sometimes there will be offers e.g. 'buy one get one half price' which brings the price down a little.
You can often find Usborne books at charity shops too, just not the newest range, but who needs the 'newest' children's books anyway?
They are expensive when at full price, just like most other books are expensive at full price. The only books I tend to find that are cheap at full price are usually not worth much.
Please don't feel guilted into buying anything from your friend. Sadly, Usborne use a MLM strategy (alongside other marketing) just like many other companies. Given the ease of ordering Usborne books online etc I doubt your friend will find it easy to earn any significant amount of money (but perhaps that's not why she's doing it). But that doesn't mean that you have to 'support her business' by buying books from her that you could get cheaper elsewhere.
Unless she is able to give you advice that you would struggle to find elsewhere; or you find value in being able to browse books in a relaxed atmosphere ('party'), compared to buying 'unseen' online, or the stresses that can be involved in book shop/charity shop shopping (e.g. screaming kids stopping you from browsing the books). In that case I find it ok to pay a bit more, as you are getting something for the extra money too.
I have chatted at length to two Usborne people. One I see quite regularly at fairs and such, and I buy from her occasionally. This lady always checks the prices her current stock is going for on Amazon and Book People, and if either of them are selling at a reduced price, she will reduce her prices too. So she doesn't knowingly charge more than you'd pay for elsewhere. Also she often has really good deals on stock she's trying to shift (to make space for new arrivals). I have a bit of a book buying habit, but I can't afford to buy books (Usborne or other) at full price. And yet I am a good customer for this Usborne dealer. She also knows her stock really well and has been able to advise me, for instance when I was looking for longer, non-scary books for an able but very sensitive young reader.
So really, if you go to this party, buy books if you think they are worth their price and you want them and can currently afford them. Don't buy 'to support your friend's business'. If you really want to support her 'business' you could get her in touch with a playgroup you frequent, or suggest to your children's school that she could have a stall at the next school fair, or some such.