My wife is actually very good with the amounts i spend. But i still use one trick to make it easier for me to introduce new stuff.
I start a few weeks in advance by mentioning what i want etc. I then proceed to "search" how much it may cost, and then tell her the RRP. I then pretend that i'm shocked just how much the "item" will cost, and say i may as well forget it.
I then spend the days leading up to the purchase, telling her how much better the "item" is, and it's a shame it's so expensive.
She then loves you for how sensible you are acting, and admires your self control.
I then phone her at work, and tell her i've managed to find it at a massive discount. And can she think of anyway we could afford the "discount" price, but if she can't, you will understand and let it go.
Being at work, she likes the fact you've consulted her first(to her friends etc). And that she now has the power to choose yes or no.
No women on earth likes admitting defeat, and "WILL" find a way to afford the item.
She has never said no (oh err!) up to now, but i would imagine a couple of days sulk would sort that.
This method has a few of plus points.
1: She cannot moan how much it cost, as "she" was the one to "decide" to get it.
2: She cannot say you don't involve her in spending decisions.
3: Since she knows the RRP(from weeks ago), you can actually remind her "how much money" you have saved!
Ps
My wife is aware of my little scam (i told her), but does enjoy the role play anyway.