Update selected data from same table column as currency
You really want to store data in fields of the correct Data Type. 
If these values are monetary amounts, then hold them in a suitable Data Type, like Currency. 

Any time you convert data from one type to another, you're make extra work for the database engine, having to do the conversion, and running the risk of losing the benefit on any Indexes you might have defined, because calling a function on a field in every row generally forces the database to Table Scan that table (unless you have function-based Indexes, but that's another story).  

Using the "wrong" Data Type also means that some functionality "doesn't work" properly.  If you're storing Numerical values as Strings, then comparing these values "goes all wrong".  

For example, in the Numeric World ... 

10 &gt; 2  =&gt;  TRUE


... as you'd expect, but in the Character world ... 

'10' &gt; '2'  =&gt;  FALSE