Having recently gone through my own wedding and being on the planning end of multiple others, the point I'm making isn't personal preference. It's the reality of how people think about weddings. The $250 number is just a number - what I really mean is that if you're going to ask far more than the norm for a wedding, it should be for a good reason. Let's say the wedding is huge, or it's extremely nice, or it's black tie, or you acknowledge the ask by getting your friends something nicer than average as groomsmen gifts. Then it could be reasonable. But forcing a group of people to spend numbers like, say $600 ($1,200 a couple) to fly to like, Eugene Oregon, plus $400 to buy a suit they don't need, plus hotels and incidentals and food - usually you're in an age group where people are having kids, recently married themselves, etc - on average at least a couple people in your wedding party are not comfortably dropping that kind of money.
Obviously there are exceptions. If everyone's older and you know they can afford the cost, or if it's a very small group or something like that it could be fine. But on average that's high, end of story. And the way you describe it - that you feel it's a big ask but the fiancee says just go for it... Well, let's just say if I had $5 for every time I'd heard a potential bridesmaid go on a tirade because a bride just HAD to get everyone to buy a $400+ dress they will never wear again - well I could buy one of those 3 piece suits.
And having just been through it - nobody is going to remember if your groomsmen had 3 piece suits or some other kind of sharp, less expensive rental. You can go down the rabbit hole paying for any absurd number of things at a wedding. In my mind, if as a couple you decide $400 suits are really important, then I'd split it with them. That's what we did when the bridesmaids dresses were a little more expensive than we'd originally thought. Or if you know it won't be an issue and you feel comfortable everyone will be happy to buy them, go for it.