Personally, I estimate that 100 coins roughly equals $.08
1,250 coins roughly equals $1.00 USD
However, artists get about 55% of revenue from coins bought in the store (30% to app fees 15% to Tapas for hosting). So keep in mind if a reader pays $5 worth of coins to tip you, you're only getting $2.75 of that. I recommend tippers donate to patreons/paypals instead of buying coins and to earn coins instead by watching ads, since artists get about 80-85% from that.
This is all just rough numbers that I've notice.