How long do you plan to stay in the house?
At $200 a year it might take you 10-14 years to break even if the current A/C does not have some problem. Do you know if it R22 or 401? R22 is getting more expensive around $100 a gallon. If you figure about $300-500 in upkeep on your old unit it might be best to swap out together if you plan on staying put for a while

Generally speaking, rule of thumb says a 5% savings with each one point increase in SEER. I don't believe it, probably close to 1/2 that.

How did you arrive at $200 in savings?

On new furnace, you will need a var speed blower or a high eff blower motor when going to a 15-16 SEER rated AC condenser and coil. If you are looking for payback justification, you will be wasting your time.

There is a marginal savings with replacing everything over just the furnace now versus replacing condenser and evap coil at a later date.

I think the better question is whether one can justify the difference between a 13-14 SEER condenser/coil combination versus 15-16 SEER condenser/coil combination. Because of manufacturer and dealer pricing, it usually is a difficult hurdle.

IMO

unit size, BTU/h àhours per year, h àenergy cost, $/kWh ÷ SEER, BTU/Wh ÷ 1000 W/kW