First and foremost need to know what part of the country these parts are in as shipping costs play into the overall price.
Knowing the year model or some of the casting numbers helps as the heads are a key component in evaluating the worth of a core engine. As a general rule and not knowing much about the engine other than it turns over, $150 is all I’ll give for a core. If it has a desirable set of heads or other wanted pieces included with it, then it obviously becomes worth more. Hearing it run or having assurances about the condition of the engine when it was running also plays into what an engine is potentially worth.