The French administration in charge of enforcing trade and advertisement regulations (DGCCRF) and the observatory of data privacy (CNIL) held an investigation on IP tracking on an unspecified set of vendors of train and flight tickets operating in France. Their conclusion was that they could not find any evidence that prices would rise when you check the same site multiple times.
I cannot find the details of the investigation, there doesn't seem to be a public report. In particular, I cannot find any information about the testing methodology or the companies that were tested (in particular whether Ryanair was included in the study).
I quote some relevant extracts from the press release (my translation).
Tests revealed practices resulting in sometimes large variations in prices: practices based on the number of seats offered or available in the desired airplane or train. This price policy (known as “yield management”) results for example in the price of a ticket depending on the date at which it was bought or the proportion of occupied seats.
Tests also revealed a practice consisting of adjusting booking fees based on the time at which the booking is made. The online shopper thus benefits from better rates when buying a ticket at “off-peak hours” chosen by the merchant.
Investigations were carried out with online merchants, then with their contractors in the domain of behavioral marketing. (…) None of the techniques that were observed took the IP address of shoppers into account as a decisive factor nor aimed to modulate the price of products or services offered to consumers.
A practice consisting of adjusting the offered price based on the site previously visited by the consumer was also observed. Thus a consumer coming from a price comparison site will sometimes see a more attractive offer, but with higher fees, resulting in no significant variation in the total price. This operation is carried out without anyone being able to determine the mechanisms resulting in variations in the displayed price.
The press release reads “the site”, singular, which hints at a decision based on the referer rather than based on cookies leading to cross-site tracking. But I wouldn't try to draw a definitive conclusion on this technical issue based on a one-paragraph layman's summary.
On the basis of this study, here are my personal conclusions:
- If you visit the airline's site multiple times, you are not penalized (price-wise) for that.
- However, given well-known yield management techniques, you are likely to see the price rise over time.
- Variations such as the modulation on the time of day make generalizable comparisons harder.
- If you browse the site in a fresh browser, you may see a different offer but the resulting price is substantially similar. This kind of makes sense as a marketing ploy — try harder to attract the comparison shopper, but sell them the same ticket in the end.
Travel Stack Exchange user Shane reports having observed the practice you describe, but sadly his testimony lacks methodology or concrete evidence.