This is one of the most complicated routes that exists for FIP travel, so I have rarely used it. Here is my understanding.
Paris - Milano is a globally priced train, which has a supplement for FIP holders. The tarifs as of 2016 were:
https://italia.voyages-sncf.eu/sites/vscitalia/files/TGVItalia-FranciaGammaTariffaria2016S.pdf (it won't have gone up much)
You need to look at the 'FIP Autres Reseaux' (ie. FIP issued by railways other than SNCF) fares. You will see they are very high, but the only upside is that all you need to get them is the FIP card, not the coupons, ie. you don't need to use a box for this journey (in other words, the system is identical to Eurostar). As you probably know, there are only TGV trains on this journey so there is no easy way of escaping these high fares. Also there are only three a day and (in my experience) they are normally pretty much full.
You may find it more economical to get to Milano via Switzerland, ie. Vallorbe or Geneva. Geneva is the best: if you travel on a TGV or a Lyria train to Bellegarde, you will only be charged the 'normal' FIP supplement/reservation of c.EUR8,50 to there, then you can go on a local train to Geneva for free (with the coupon). This is much cheaper than a FIP fare on the direct trains from Paris - Switzerland (Lyria), the higher fare only being charged in effect to cross the frontier as these trains operate as normal domestic TGV in France. You will need coupons for FR, CH and IT to do this and the Swiss will charge you c. CH10 for a reservation/supplement on the Eurocity trains from Domodossola to Milan.
Alternatively, you could book a TGV supplement to Chambery or Modane (if the train stops there), then an FIP fare from there to Milano for c.EUR 46 in 1st, c. EUR 35 in 2nd, ie. a split ticket. The complication here is cancellation of the ticket (ie. 'composter'). It seems a little unclear whether you can 'composter' the ticket from Chambery/Modane onwards at Paris, or whether it has to be done at the origin station only. None of the conductors I have met in France seem at all concerned about this when I have done it but I have feeling the official rule is that ticket must be composter at the origin station. So you could travel one train earlier to Chambery so that you board the TGV to Italy at that station.