Would you outright sell the Ruger? Whenever you're considering a trade, that's where I start - would you sell it? Meaning you simply would not have it in your life. If you're willing to sell that particular Ruger, but only currently want to have a back-up 22LR pistol behind your Suppressed Ruger, then you're justifying for yourself the ownership of "something," and the Buckmark might make sense - however you'll find yourself with a different feel than your primary, which might not be of consequence, or might be a deal breaker...
If you can decide whether you would sell that Ruger, or if you even need a back-up, then a guy can look at what the market value on both items involved in the trade might be. Like others have mentioned, the cash is going the wrong way, as your Ruger is worth more than the Browning.
Personally, new price on a Buckmark should be around $525 for grey/silver/stainless models, figure $450 used, suck off that $75 cash differential, and you end up with $375 - that's assigning $450 to a used Buckmark and $375 to a used Mark II. That's a GREAT deal on a Mark II but NOT even a good deal on a Buckmark, so again, you're not really giving your pistol enough value. If I were wanting a Buckmark just to try it, used at $450 or new at $525 would not be a high enough cost to preclude me from just buying it. If I found I liked the Buckmark and no longer wanted the Ruger, I'd sell the Ruger... If I found I preferred the Ruger, I'd resell the Buckmark... If I found I didn't need either, I'd sell both... If I later realized I did, indeed, want either of them, then I'd buy it, because they're readily available on the market... A decade later if I wanted one or didn't want one, I'd buy or sell it... We don't swear before God, 'til death do us part when we buy a pistol - it's a commodity product, not a marriage.