You should check your avg. CPA on display or search and set the CPA for the smart display accordingly. +50% if you take in consideration the avg. CPA from search and +20% if you consider the avg. CPA of the display. The budget should be around 10x higher than the CPA you set. Additionally, if you do not have enough conversion data and you want to pay for the conversion, it might not work. So, it would be a good practice to pay for clicks at the beginning with a lower CPA.