Sure, weddings themselves can be pretty pricey once you start inviting all your aunts and second uncles but getting engaged doesn't need to be. Whilst the average spend is pretty high (between £600 and £1700 depending on which survey you believe), it is perfectly possible to get a cheap engagement ring for a similar price or even less than an XBox One.
As I write this, a new XBox One will cost you around £230, so that's the budget I'm going to work to.
First stop, I'd suggest is Amazon where you can (at the time of writing) pick up a 0.33 carat diamond engagement ring for £199 (the price varies depending on your ring size, but most sizes are within my £230 budget). That's not a huge diamond, but it isn't particularly small either so it will look perfectly respectable for most people.
Of course, size (or rather carat/weight) isn't everything with a diamond ring. The quality - or grading - of the diamond also matters and for a low-end budget you won't get the highest graded rings. My (somewhat controversial) view is that for most people, the differences will not be especially noticeable until you either use a magnifiying glass or you start putting vastly different diamonds alongside each other.
If you'd rather buy from a jeweller which has high street stores (making returns easier) then take a look at Argos. Their prices do fluctuate with sales and promotions, but I've seen 0.25 carat diamond rings on offer at around £225.
If you're feeling a bit more adventurous, I'd consider buying from a US jeweller. You'll have to pay customs fees, but the prices at a retailer like SuperJeweler sometimes make it worth the hassle (check out the blog on importing diamonds for more on this).
Finally, I'd look at the second hand market. With an XBox budget you'll be able to get something which originally retailed for around £500 - £1000. It's up to you whether you (or your potential spouse) would be happy to wear preloved jewellery. I'd start by searching for diamond engagement rings under £200 on eBay.
So, if the cost of the ring is putting you off - but you think you can afford an XBox One - then I'm afraid it's no longer a legitimate excuse...
Happy hunting, I hope she says yes.