It's around 800 to get the 22 and convert it up, but the regular without a conversion run 800 or more. So I really don't think that's a good idea.
Last I checked, the coupon included in the "22lr Classic X-change" kit allows you a centerfire upper with only one mag for $399. So you might be into it for close to $1000 by that point, and you'll get exactly the centerfire that they've decided you'll get. With one mag.
Do it the other way around, and you can get the centerfire setup that you prefer (they can be found for all points of $600 to $1500+ depending on what you want).
I paid too much for a Classic P226 22lr, and too much for a centerfire conversion that I waited 6-8 months to receive from Sig. Great gun otherwise. But my advice is that you figure out exactly what caliber and design (Equinox, standard, MK25, night sights or not, 40sw or 9mm, etc, etc...there are a ton of different designs in the P226, and you can even spend $500-600 on an excellent used centerfire) you prefer before jumping in. And when you make a decision, investigate the online buying options. I've seen both upper kits (rimfire and centerfire) available for $100-200 less than what Sig charges, and they'll ship out next day...as opposed to Sig's lengthy wait.