Well pay level in its self does not determine if you are a net contributor, tax and benefits received may in a crude way. However benefits are often calculated in a simplistic way, such as "benefits" received JSA, housing benefit tax credit. These are simple as received in clear numbers. Although housing benefit does not consider if living in subsided house such as council or associate, this uncalculated benefit.
There is also other things such as education, health care which possibly less well costed.
( also you have to convert to the market rate, to cost which is possibly just an estimate if no market its what it would cost if there an effect market, so cant just use what things cost or otherwise inefficiency or government manipulated costs either such as through supply regulation skew the true picture think of housing costs on benefits as classic example)
However something things no one even attempts to cost.
Imagine a Russia oligarch moved to the uk, and takes no tax credit, what are the security cost worth to begin with the more assets you have the more subsidized you are in effect all things being equal? What if they take legal action against another what are the cost subsides worth? Court costs are highly subsided. No one considers these in the calculations, people choose the accessible figures but these may not always show the full picture.
What if someone works protected industry such as law, or state employment or union employment and paid above the "market rate" the figure they are paid is in part a subsidy this is not considered. In effect government regulated employment such as law is effectively like a tax a cost through regulation. Yet no one measure this so many variables no one considerers. Someone may work for 18k in food processing a competitive largely unregulated area of employment and make the contribution greater than say a protected or government job if you discount the subizededs from protection or political overpayment.
What is the market rate for an MP? the criteria they like to apply on the lowest people? Well as people would probably pay to be an MP its in the minus most likely. However one may argue well what of standards what standards?, in the classic thought you get what pay for , but what is an uncompetive industry such as politics, where selection controlled by parties not a great deal.
Someone would have to apply a multiplier benefits to GDP bring from the persons input employment income does not for reasons such as stated above, Tax and benefits would have to be adjusted to real rates Guess this and would have to adjust benefits first to a guess of market rates. As such manipulated economy would be difficult instead get simplistic nonsense using to little data on government accessible unadjusted figures.
What if someone has a non-job in government ( or protected through reg same diff) a job that does not provide any benefit to anyone just a cost, that pays 19K and someone else has a job at 18k in non government supported job that creates 3 times their salary in that GDp, who gets deported? May be Australia does it on wage but who cares what they do its worse to cycle there than here is it not?
There is something unBritish about sending this woman back the rest is just in my opinion BS after the fact to justify a silly policy dodgy arbitrary figures that don't really mean anything