Sixbid's archive site is free and okay as a first, cursory look: https://www.sixbid-coin-archive.com/#/en I often will look at multiple sources, like: what is offered on vcoins, what sold by CNG, I search ACSearch to find coins in similar condition and then use Sixbid to find the sale and results of the auction (I don't have a premium ACSearch account... I'm cheap). Only then do I feel more comfortable with estimating the value range of a coin.
After all this time, I finally gave in the other day and bought a 1-year subscription to the premium version of ACSearch. Given that the cost is less than $100 annually, and my average spending on ancient coins every week or two equals or exceeds that sum, I decided that it made sense. In fact, I think it would make sense even for someone who spends only $100 per month on average. Especially compared to Coinarchives, which I believe costs more than $600 annually for a subscription.