Surprisingly, many appeals can be very inexpensive. For instance when the appeal depends entirely on one clearly defined issue of law, that has already been well briefed, it may cost very little to appeal.
Other appeals – such as claims that the verdict was against the weight of the evidence -- typically require both the printing the entire trial record and extensive analysis and briefing. Such appeals are relatively expensive as they can require extensive amounts of lawyers’ time. They also turn out to be less successful.