What is the cost of converting one gram of mass into pure energy?

$$E=m{c}^2$$

Using the SI system of units:

Physical quantity	Unit of measurement	Value
E - energy	J - joule	x
m - mass	kg - kilogram	0.001
c - speed of light in vacuum	$\frac{m}{s}$ - metres per second	300,000,000

The speed of light in vacuum, commonly denoted c, is a universal physical constant that is exactly equal to 299,792,458 metres per second (approximately 300,000 kilometres per second)

$$E=0.001kg\cdot {\left(300000000\frac{m}{s}\right)}^2=$$

$$=0,001kg\cdot \left(90\cdot {10}^{15}\frac{m^2}{s^2}\right)=90\cdot {10}^{12}kg\frac{m^2}{s^2}\left(J\right)\left(\left(Ws\right)\right)=$$

$$=90\cdot {10}^{12}Ws=\frac{90\cdot {10}^{12}}{3600}Wh=25\cdot {10}^{9}Wh=25GWh$$

$$1gram=25GWh$$

$$1kg=25TWh$$