Clearly marking your exits is a necessity. Not only can patrons find their way out of your business more easily, but you will also avoid the fire department fining you for not marking your exits. (It is a fire safety hazard when you do not post exit signs above doors.)
However, it is understandable that you may not be able to afford any electrical or neon exit signs. That is okay because the law allows signs that clearly mark exits and other types of signs that use other means of "lighting" up. The following short list of exit signs is still acceptable to fire departments and building code inspectors.
LED-Lighted and Battery-Operated
There are exit signs that incorporate LED lights, which are the brightest, smallest, and least energy-demanding of all bulbs, and that incorporate battery power. Since you are using both batteries to power the sign and LED lights to light it, you will not have to hook these signs up to electrical power boxes. Better still, the lights will stay lit when there is a power outage. The LED bulbs will take more than a month (roughly) to burn out the batteries. The bulbs themselves can last months.
Color-Changing with Heat/Cold
Paint that changes color when it detects heat or cold can draw people's attention to the exits. The hotter the room, the brighter the letters in the word "EXIT" appear, alerting people to the heat (or a possible fire) in the room. Extreme cold reverts the letters back to a different color but still makes the exit signs obvious.
Photoluminescent Exit Signage
These signs glow in the dark. This is also ideal in the event of a power outage. The photoluminescent paint keeps exit signs glowing without the need for electrical power, and they continue to glow even when your business is closed for the night. All they need is a little exposure to light while your business is open.
If you have an exit that is not well-lit, you can either install a ceiling light or you can rotate these signs so that each gets enough light to glow properly when your business is open to customers. (When you already cannot afford electrically powered exit signs, you opt for sign rotation to keep these signs "charged up.") You could also place a stick-on/up battery-powered light in front of the photoluminescent exit signage that is above the door where there is no good light source.