Standard form in TeX?

If you need to typeset actual numbers (and not symbols as in your example), you could use the \num macro provided by siunitx. It allows you to type \num{2e3} to output , both in math and in text mode.

When multiplying numbers in exponential form, you could reduce the spacing within the numbers, using tight-spacing=true:

\documentclass{article}
\usepackage{siunitx}

\begin{document}

\sisetup{tight-spacing=true}
\[
g = \frac{\num{6.67e-11} \times \num{6e24}}{6400010^2}
\]
\end{document}

I know many will disagree with the following but nonetheless this what I used to use:

\newcommand{\sn}[2]{\ensuremath{{#1}\times 10^{#2}}}

And I use it like this: \sn{y}{x}. Here is an example:

\documentclass{article}
\usepackage[dvipsnames]{xcolor}
\usepackage{amsmath}

\newcommand{\sn}[2]{\ensuremath{{#1}\times 10^{#2}}}

\begin{document}
\begin{align*}
(\sn{2.25}{\textcolor{Cerulean}{4}})(\sn{7.5}{\textcolor{OrangeRed}{6}})    &= \sn{(2.25)(7.5)}{\textcolor{Cerulean}{4}+\textcolor{OrangeRed}{6}}\\
                                   &= \sn{16.875}{10}\\
                                   &= \sn{1.6875}{11}\\
                                   &= \sn{1.7}{11} \quad(\text{2 s.f.})
\end{align*}
\end{document}

you can use {\times} instead of \times, the former does not add the additional spaces