The cost here is not a cost to the Exchequer Bigfella.
When the MW goes up, people are taken in to tax who were not previously and people are taken out of benefits, or will receive less benefits.
So the cost is carried by the employer and the government should see an increase in revenue.

The cost related to curbing migrant benefits will have little if any financial benefit to the exchequer.

The child benefit part of this deal is that where child benefit is currently paid to (for example) a family from Hungary, instead of that being paid at a UK rate, it will be paid at a reduced rate, based on a comparison between the standards of living in the 2 countries.
So it will save a little, not all.

That being the case, you have to take in to account that Germany, Austria, Luxembourg and perhaps a few other nations actually have a higher standard of living and subject to the agreement they might actually get increased benefits.
Many of the countries, such as France, Netherlands, Belgium.... are hardly any different.

So the financial saving is likely to be peanuts in the scheme of things and there will be an increased cost to administer.

The effort made on this, is because it is seen as a further potential draw for EU workers to come here. The draw of the increasing minumum wage will probably be more of a draw, than the decrease in potential benefits.

So nothing is likely to change as a result of this part of the deal.

On the wider benefits, measures can be brought in if immigration reaches "exceptional" levels. That doesn't seem to be defined,it is not a complete reduction and are tax credits counted as a benefit anyway?