How to Get and Set Properties during script assertions?

Since 'price' is variable declared here. You cannot pass it as String. Please change the code to :-

messageExchange.modelItem.testStep.testCase.getTestStepByName("Estimate").setPropertyValue("EstimateItem1Price", price)

If it doesn't work. Please try this way also:-

testRunner.testCase.getTestStepByName("Estimate").setPropertyValue("EstimateItem1Price", price)

And also, please change price variable to price1 or anything else. You already have price as jsonObject in your Json.