It just may be as simple as needing to loosen up the buckle/strap or wear thinner socks. If your socks or shoes are too tight or too thick (don't forget that your feet will swell once they're warmed up) then the blood circulation is cut off, hence, numb toes.
You also may want to have your LBS double check your cleat position. Even a Millimeter will make a big difference over the long haul all the from your toes to your hips and knees.
If you do end up needing new shoes, check Pricepoint.com for some Lake shoes. They usually have some on sale for about $20-25. They're not nearly as stiff as the Shimanos, though.
Good luck!