The only socketed (upgradable) laptop (mobile) Intel CPUs are the i7 models ending in MQ. They are generally only found in gaming laptops, and far, far above your price range.
Is there a particular reason you want the CPU to be upgradeable? Ever since we hit the thermal frequency limit around 2005, CPUs haven't sped up that much. Most of Intel's and AMD's R&D has been towards getting more instructions done simultaneously (e.g. hyperthreading), adding more cores, and reducing power consumption. For example, at the same clock speed, Skylake is only about 25%-40% faster than Sandy Bridge, despite 5 years having passed. In the 1990s, CPUs doubled in speed every 1.5-2 years. Most of the advancements have been in reducing power consumption. Skylake uses about 1/3rd the power of Sandy Bridge.
This has led to a curious phenomenon where newer versions of software run better on older hardware. For example, Windows 10 runs better than Windows 8, which runs better than Windows 7 on old hardware. What's happening is the software makers can't add features, while relying on the speedup of CPUs to compensate for the feature making the software run slower. Consequently, they've been putting more effort into optimizing their software to run more efficiently. The result being that newer software runs better on older hardware than software which was contemporary to that hardware when it was new.
So bottom line is, barring any huge technological breakthroughs, any mid- or hi-end computer you buy today should continue to run "modern" software for 5-10 years. And you shouldn't need to upgrade your CPU.
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