NT merchandise boost varies greatly based on the salary of the player.McArthur used to give me a lot more merchandise than Dell'Asta did.The bonus for home grown NT players is a fair bit higher than internationals.
Merchandising income comes from your fans buying team apparel. Jersey sales are affected by a combination of factors. One factor is whether you have a winning team. Nobody likes to wear the jersey when you're losing, do they? Other factors include how many domestic players you have on your team, and also how many were drafted by your own team, as fans like supporting local players more than imported talent. This is weighted by the overall skill of the local players, so stocking your team with local bench warmers won't do you much good. Similarly, fans from around the country are particularly enthusiastic supporters of members of their national teams, and even more so when their national team is a successful one. So, if in a given week your player played in a national team game, fans from around the country will buy their jersey, typically in a quantity proportional to their salary. National team jersey sales will vary depending on the strength of the national team, but players playing in their domestic league tend to sell considerably more national team merchandise.