How significant is antenna drag on light GA aircraft?
From an aerospace engineering perspective:

The drag coefficient ($C_D$) of a round antenna is can be estimated by around $.5$ depending on shape, while for an streamlined shape it's around $0.045$.



(Source)

Drag is defined as:

$$D=\frac12C_D{\rho}V^2A$$

Thus, say you have a 0.01 m wide antenna 0.25 m long, for an aircraft moving at 50 m/s. At sea level conditions, air density $\rho=1.225 \mathrm {kg/m^3}$. Say that $C_D=0.5$. If my calculations add up, it's around $1.9 \mathrm{N}$.

Say you have the same antenna, for the same aircraft aircraft moving at 50 m/s. Air density is $1.225 \mathrm{kg/m^3}$, but this time the antenna is streamlined with $C_D=0.04$. This results in a force of only $0.15 \mathrm {N}$.

(Updates: Originally the formula included $\frac12$ twice which was wrong, and that the force was given in kg, rather than newtons; my apologies.)