As I recall, it's $40 for 3 days.
Although the ruins are spread over a vast area, the main sights are fairly close together. The highlights are Angkor Wat, Angkor Thom, and Ta Prohm (the overgrown one). After those, the outlying sites pale by comparison.
At sunset, EVERYONE heads up the hill. Worthwhile, but it will NOT be a solitary experience.
No need for a guide; a map/guidebook is sufficient. No shortage of transport on site.
Don't miss a cruise on Tonle Sap.