How is the tetrachloridocobaltate(II) anion stable if its crystal field stabilization energy comes to be positive?
You didnt put the right values. Higher orbitals have energy value of $+0.4 \Delta_t$ and the lower orbitals have the energy value of $-0.6\Delta_t$.

This complex has a coordination number of 4, so it can have tetrahedral or square planar geometry. Tetrahedral complexes are favored where attainment of regular complexes is important. For the tetrahedral complexes $\mathrm{d^0, d^2, d^5, d^7}$, and $\mathrm{d^{10}}$ the configurations are regular.

In tetrahedral complexes, mostly $\Delta t &lt; P\ (\Delta_t \approx 4/9 \Delta_o)$ and hence they generally form high spin complexes.



The splitting will be into $3\ \mathrm{t_2}$ (not $\mathrm{t_{2g}}$) higher energy orbitals and $2\ \mathrm e$ (not $\mathrm{e_g}$) lower energy orbitals. The effect of the ligand wont matter here because the value of $\Delta_t$ is small. So the filling will be same for strong field and weak field.  

$$\begin{align}\text{CFSE} &amp;= +0.4(3) -0.6(4)\\
&amp;= -1.2\end{align}$$