Finding Nemo: Doing the sums
Kiss kiss, goodbye.

This was a very chaotic and hastily thrown-together trip.
I only found out on the Wednesday that I'd need to travel on Friday, which came as something as a surprise as I really hadn't expected to do any washing before the weekend. Last-minute tickets on French trains were suffering from the SNCF's hateful "dynamic airline-style" price-gouging, but with a spare four-day pass in reserve I was able to drop everything and sashay off on a sparkly little train adventure on a whim.
Things would have been different if there had been no pass-holder fares available between Angoulême and Strasbourg, although I'd probably have figured something out via Paris. Otherwise I'd have looked very stupid.
What a fun fortnight it turned out to be: trains, frocks and sparkles, beer, train-sausage, falafel, and fabulous. According to the Interrail app, my journey took me only 2,777 kilometres, so I haven't beaten my Easter trip for distance travelled on one four-day pass. This means I need another one, because obviously I have failed my audience.
tl;dr
Last-minute travel around Europe, especially if it involves France, is significantly cheaper with judicious use of an Interrail pass.

When I looked, the only direct TGV from Angoulême was showing at 215€, but there was still a pass-holder fare available which I snagged for 10€. I could have gone via Paris, but this would have incurred a reservation fee for the extra leg. Once I'd booked and paid, the train showed as full on SNCF Connect.
An onward ticket to Basel would not have been expensive at 44.60€. I didn't need a reservation and the train was quite quiet. In Basel, I was able to use my pass on the S-Bahn to get to my hostel, where I was given a Basel Card entitling me to free public transport for the rest of my stay, but I did have to ask for it at check-in.
The ticket to Eurovision also entitled me to a free return journey on public transport to and from the venue.

I didn't make any plans for this journey which, given the number of people in Basel at the weekend, was probably slightly reckless. I took my chances on lunchtime services and found seats on both.
The cost of the journey from Basel to Zürich was about 72€ after conversion, and the onward journey on the tilty-but-meh SBB train to Munich with needlessly tiny toilets was about 100€. By using my pass on the S-Bahn, I saved 6,10€.

This journey was more stressful than it needed to be but ultimately nothing went catastrophically wrong and I got to my destination unruffled.
When I made my seat reservations a few days in advance, tickets from the DB were 218€ for the whole journey. I paid 6€ for the initial reservations and then a second 6€ to secure a seat on the alternative journey from Cologne to Brussels, which I might try claiming back from Deutsche Bahn as a matter of principle.
In the heat of the train cancellations, last-minute Frankfurt to Brussels was showing as "from 367€" in DB Navigator.

I didn't make any reservations for the first two trains of the day. Without a pass, Brussels to Lille in first would have cost 29€ – and this at any time of day because the Belgians are sensible; rail tickets are calculated based on a fixed price depending on the distance travelled.
The onward TER (second only) connection to Paris was 43€, but again I paid nothing as it was included. Obviously this was slower than a TGV but you gotta speed it up, and then you gotta slow it down. In Paris, I bought a 2,50€ métro ticket but the barriers weren't working so I wasn't charged.
Tickets for Paris to Limoges are rarely excessively expensive. I booked the last available seat in coach five – which was conspicuous by its absence on the day – three days before travel, and would have paid 85€ for the ticket. The reservation fee was 20€. The SNCF refunded 4€ of this – perhaps for the emotional distress.
Had I booked earlier, I'd probably have got a solo seat in coach four and possibly a lower reservation fee.
Limoges to Périgueux on the TER always costs 21€.
Playing with numbers.
Good lord. Well done getting this far. It doesn't get any better, trust me.
A normal person would probably have known more than two days before departure that they were going somewhere, so we can assume that advance booking prices are certainly better than those listed here.
While this method is hardly scientific, looking now for a return trip to Basel from Angoulême next weekend, the best return fare in first is roughly 440€. In second, the best fare is 368€. With a Carte Avantage, those would come down to 312€ and 222€ respectively (after spending 49€ for the card).
My promo four-day pass cost me 306€, so even if I had wasted two days of travel just by doing the return trip, I'd have been more or less quids in. It was the onward journey to Zürich that made the pass pay off.
Adding the extra frivolity of coming home via Gröbenzell and Brussels pushed my total transport spend up just 12€ for the seat reservations. The excursion to Waterloo cost 8€ return.
- Cost of four-day first-class pass (15% off): 306€
- Reservation fees and last-minute Wa-Wa-Wa-Wa-Waterloo: 50€
- Total cost of transport without pass (estimated): 835€
- Total costs of transport using Interrail pass: 356€
- Savings: 480€
The following Austrian cities are definitely reachable from Angoulême in one Interrail day: Graz, Vienna, Salzburg, Linz. Just saying.
Denmark was robbed.