It's hard to say precisely, because there were several different RG370 models made in the mid-to-late 90's that have material differences — some are made in Japan, others in Korea; some are made of Resoncast (essentially MDF); etc. In addition, a major portion of the value of a 20+ year old guitar is in the condition of the individual example. Also, values are impacted by local market conditions, a guitar that would sell for US$200 in Los Angeles might bring twice as much in the Philippines, for example.
In general, I wouldn't want to pay much more than US$250 for any RG3xx series guitar. I can find RG5xx guitars for just a little bit more than that which are generally much more stable, so it caps the value of the lower end models.
If you have a specific guitar in mind, feel free to share some pictures or a link to the sale listing and I'll try to give you a better idea on the actual value.