If you are under Software Maintenance Service then there is no cost. If you go to https://www.mathworks.com/downloads/web_downloads/select_release?mode=gwylf and it offers to let you download R2016b then you are fine.
If you are not under Software Maintenance Service, then your lowest cost option is probably to back-pay the Software Maintenance Service. Years ago the Mathworks policy was that you could back-pay up to 4 years; I do not know if that is still the case.
If for some reason you are not covered under maintenance and cannot back-pay the maintenance then you would need to re=purchase MATLAB (and any toolboxes you need) at full price. There is no "upgrade price" as such.
