Strats routinely come with humbuckers these days (known as "Fat Strats"), so there's nothing disgraceful about it - in fact I prefer mine with a Floyd Rose which is even more of a sin, but I prefer a tremolo that stays in tune. :D I'd ask for $449 (no harm in asking), but be prepared to sell...
With the case, $400-425 isn't unreasonable. Like I said, a new one can be bought for only $75 to $100 more without a case, so it will depend on how much added value your buyer sees in the upgrades.
It really depends on the buyer. If they like the upgrades, it is worth more, if not, less. If it were stock, you might be able to get $350 for it, but with new ones being available for not a lot more, it is tough to say. A lot would depend on how well it is set up too. Does it have a case? I...
These retail for around $450-500CAN new. The color and fretboard are standard issue. The aftermarket parts are probably another $50-80 new. There is not much special about the Mexis since there are hundreds of thousands of them (even though I personally prefer them to the US made ones), so...