Roseformeplease - Most people I know spend more than £150 for a wedding dress. At least 5x that, and I know many - me included - who spent at the very least 10x that.
And £35 for a prom ticket is the norm (or even the lower scale of things - going to a theatre in London can often cost more than that). My prom was over a decade ago, and as I recall, it actually cost a bit more than that - so if adjusted for inflation, it would be much more expensive than the ticket price the OP has to pay. Granted, I made the dress myself (sewing, appliqué, etc.) but the materials alone cost something in the region of £130 in today's money. Why I didn't just buy a dress? I couldn't find the style I wanted (early century / dinner in the Titanic style), and it would have cost much more than £130 if I had.
Pootles2010 spent £70 on her prom dress and limo alone. Not sure how much the ticket was, but let's say her total expenditure was roughly £90... back in 2002. Adjusted to inflation, that would have been about £125 in 2012. Calandarbear already admitted she spent £160 on the dress alone back in 1999.
If I shop for "special occasion" dress these days which - as it happens - I am, I can't find a lot of dresses I like below a certain amount of money. No price snobbery there - if I challenge myself shop price-blind I often choose the most expensive dress, and end up not buying anything at all because nothing can compare to that dress.
As a parent, it's easy to say "it didn't cost that much in my days", but really... take into account inflation, and how things have changed. Back in the olden days, you may have had a prom in the school gym. These days, it's hotels, etc. - venues where you often pay £50+ per head for a rather simple wedding (and much more if you want a more glamorous one).
Anyway, if you want to have fab dresses that look like they should cost more, I would second tkmaxx and eBay. There are loads of Chinese dresses on eBay that look like they should cost £££'s, but because it's made in china and all that, it actually isn't that expensive. In the Far East, having your special occassion dress custom-made by a seamstress is normal. Only wealthier types can buy Western Designer goods, and they often are made there anyway. Some eBay sllers often make the dresses made-to-measure, too.