An "introductory" offer is an offer of a product or service that you have not previously provided.
Clearly, it is impossible to make a price comparison in any "introductory" offer because you would not have established a regular selling price. If you can substantiate it, you may make claims such as: "Sells elsewhere for $X - our price $Y" or "RRP $X - our price $Y".
The offer should run for a restricted "reasonable" time only, after which it would clearly be misleading to continue to describe it as an introductory offer. The law does not define "reasonable" time for such offers, but in most cases weeks rather than months would be considered reasonable.
To make it clear to potential purchasers, it would be advisable to state the date on which the offer expires.