Well, if an opamp that costs $2.57 from Digi-Key in single quantities is too expensive, then I doubt I can help you any further.i think it's a little expensive for my application which generate mv signal .
You could use a offset nulling circuit, which introduces a compensating offset at the input. The offset varies w.r.t. temperature, which is specified in the datasheet, which is usually quite small in precision opamps.i found this chip MCP6022 http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en010439 but it's offset is .5 mv , Is this .5 mv should appear in output or it will be from 0 to .5 mv and what does affect this increment ?
Is it constant over some time so i could subtract it of output if it is known ?