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e.g. as below
It looks to me like you could set the $39.99 first, then the /SQ FT, and kern the latter back to sit under the superscript.
The /SQ.FT is not subscript. Make the 99 superscript. Make the /SQFT smaller type and normal, not super script. Place your cursor at the end of .99 and set the kerning to -1000 or whatever.
To go a little further, you can do this in one shot with GREP styles.