I got my Nuvi 500 for $170 refurbished on Amazon. Another $40 or so got me the Ram mount setup.
There aren't many waterproof GPS units around in that price range. The only reason I can see to buy a Zumo is for the bluetooth out feature. The 500 can be hacked to provide an earphone jack very simply.
If waterproofing isn't important to you, then you have about a million choices for less than $200.
Young enough to know I can. Old enough to know I shouldn't. Stupid enough to do it anyway
'13 Duc Multi GT, '10 Vstrom 1k, '10 705cc KLR