Square a Number my Way

05AB1E, 7 bytes

þSDg×+O

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Explanation

þSDg×+O Implicit input
þ       Keep digits
 S      Get chars
  D     Duplicate
   g    Length
    ×   Repeat string(s)
     +  Add (implicit input added to all elements)
      O Sum

Jelly,  13  12 bytes

fØDẋ€L$ŒV+VS

A monadic link accepting a list of characters (a well-formed decimal number, the single leading zero being a requirement for -1 < n < 1) and returning a number.

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14 bytes to accept and return numbers (input limited at +/-10^-5 by ŒṘ): ŒṘfØDẋ€L$ŒV+⁸S.

How?

fØDẋ€L$ŒV+VS - Link: list of characters         e.g. "-0.45"
 ØD          - yield digit characters                "0123456789"
f            - filter keep                           "045"
      $      - last two links as a monad:
     L       -   length (number of digit characters) 3
   ẋ€        -   repeat list for €ach digit          ["000","444","555"]
       ŒV    - evaluate as Python code (vectorises)  [0,444,555]
          V  - evaluate (the input) as Jelly code    -0.45
         +   - addition (vectorises)                 [-0.45,443.55,554.55]
           S - sum                                   997.65

Haskell, 59 56 bytes

f s|l<-filter(>'.')s=0.0+sum(read<$>(s<$l)++[c<$l|c<-l])

The input is taken as a string.

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How it works

l<-filter(>'.')s      -- let l be the string of all the numbers of the input string
f s   = 0.0 + sum     -- the result is the sum of (add 0.0 to fix the type to float)
   read<$>            -- turn every string of the following list into a number
   s<$l               -- length of l times the input string followed by
   [c<$l|c<-l]        -- length of l times c for each c in l