You are asigning the generated cursor to a numeric (currency) value, you need to assign the value stored in the field of the cursor generated. Change your code to this (notice the lines with ySum):

local ldLastYear, lcThisClient, curLastYearSales

***

ldLastYear = YEAR(DATE()) - 1
lcThisClient = thisform.cboClientName.Value


SELECT SUM(Cdm30wrk.wrk_tlamt) as ySum;
 FROM  classicdb!cdm30wrk;
 WHERE Cdm30wrk.wrk_stat = "A-ACCTNG" = .T.;
   AND Cdm30wrk.wrk_page = 1;
   AND ALLT(Cdm30wrk.cmp_name) = lcThisClient;
   AND YEAR(Cdm30wrk.date_updt) = ldLastYear;
into cursor curLastYearSales
thisform.pgfClientDetails.pagAccountDetails.txtLastYearSales.value = curLastYearSales.ySum