Something like this? Firstly, a loop to demonstrate we're getting the correct value on every day
<cfset cost = 50>
<cfloop index="day" from="21" to="120">
<cfset extraCost = ((day-1) \ 30) * 34>
<cfoutput>
#day# days, #extraCost + cost#<br>
</cfoutput>
</cfloop>
Then probably something more like the code you'll need
<cfset days = 61> <!--- or whatever --->
<cfset newCost = (((days-1) \ 30) * 34) + 50>
<cfoutput>#newCost#</cfoutput>