Well, I'll actually give you a serious answer.
Your keyboard is worth about $75 depending on the condition. Does the LCD screen still display nice and bright? Do you have any of the "Music Cartridges" that were made for this line of keyboards? Is the plastic music stand attachment still intact? How much was it played? Did it get cigarettes smoked around it?
All those things will be factors determining worth. You can also find all you need direct from Yamaha here: link
You could actually sell this keyboard and find a pretty nice used board for maybe twice what you sell it for.
Good "Pro" entry level synths would be the Roland XP10, Alesis QS6.1, Korg X5. You can pick those up used from around $150-$200 and have some pretty decent general sounds from those.
Alesis made a great little sound module called the QSR and three smaller versions: NanoPiano, NanoSynth, & NanoBass. You can get any one of those and plug 'em into your Midi Out port for less than $150.
You would notice the difference in quality of sounds within the first few notes that you play.