bags on budget airlines: bundle vs. a la carte

Check with them. With discount airlines, the cost of checking a bag at the time of flight can be 2-3 times what is charged in a bundle or on booking.
Edit: Here’s a link to Ryan’s Schedule of Fees. Here’s what they say for how much they’ll rip you off :

• Fee varies depending on the route and travel dates selected. Non-priority customers who have not added a bag to their booking can still purchase a 10kg Check-in bag at the airport bag drop desk for €/£ 35.99. Non-priority customers who bring their 10kg Check-in bag to the boarding gate may check it in against payment of a fee of €/£ 46.00 - €/£ 60.00.*

Absolutely, buy now. Pre-Covid, for payment, they used to require cash at the counter rather than CC because so many people would try to void the transaction when the CC bill came.

To clarify, I don't mean to wait to add a bag until I get to the airport. I would do it immediately after booking the Basic fare.
Before you reserve the flight, Ryanair doesn't show a non-bundled price for adding a carry-on bag. So the question is: what is that price compared to the price of a bundle that includes a carry-on bag?

They will show you the price for just adding the bag right at the end of the booking process, before you pay. They strongly push the bundles presumably because they get more of your money that way.

I always opt to just pay for the luggage I need. I find there is very little benefit in paying to book specific seats or for speedy boarding. This may not apply but Ryanair do insist on you paying for seats if you are travelling with children.

There are several good discount airlines -- look around you may have better options than RyanAir. And buy the bundle ticket. It is cheaper.

First, not buying the bundle I seriously doubt will save you any money and probably cost you more.

Second, be aware of what you are buying. Ryanair has three types of bags that are paired with different ticket bundles. And the sizes are a bit oddball.

Small Bag 40 x 20 x 25
Cabin Baggage 55 x 40 x 25 and 10kg
Checked Baggage 10kg or 20kg

On one flight I checked
Basic got you a Small Bag for 25.000ft
Regular got you a Small Bag and a Cabin Bag and a seat reservation for 15.000ft more
BUT NO CHECKED BAG
Plus got you a Small Bag and a Checked Bag and a seat reservation for 16.000ft more
BUT NO CABIN BAG!
Flexi Plus got you the same as Regular with a few perks for checking in and boarding and flight mods and cancelations for 38.000ft more

WIzz is equally as confusing. But I like both. They get the job done and if you travel light, the prices are good.

It is cheaper to just buy what you want. There’s no way a budget airline is giving you anything for free. If you are happy to go without speedy boarding and a seat of your choosing then you will pay less. You do get an option to just pay for your bag.

You won’t be able to find out costs until you go through the booking process because the prices vary across routes.

I always just get the Plus option, which allows airport check in, an assigned seat, and a 20 kilo checked bag.

You keep saying carry-on, but if you are looking to take your "carry on size" bag into the cabin, it will need to be under 10 kilo and meet the size requirements. When I travel, I am always at that ragged edge of 10 kilo (especially after traveling a while) and simply do not want to worry about being over. Checking a bag carries little risk since the flights are point to point, unless you are stringing flights together.

An addition: I notice above no one has mentioned that all budget airlines, and most European carriers, are strict about size and weight limits. I have flown when everyone was required to weigh their bag, if you check-in at the airport, you always get your bag weighed, and at the gate they look around and spot suspect bags to be weighed or put into the sizer. If you are over, you wind up paying a large fee. Better to plan to the safe size, people who risk it to go cheap, wind up getting burned, the cost of the plus bundle is still a cheap ticket to get where you need to go with no worry or risk. I have flown Ryan Air a number of times, every time a good experience, but every time, I see someone else not have a great time due to an oversize bag.

Thanks all, especially Helen for her very specific answer. I'm confident between my "personal item" (small backpack) and carry-on, my family and I can each meet Ryanair's size and weight limits, and we'll bring a scale and tape measure to make sure before each flight. We do always travel light and avoid checking bags unless absolutely necessary, e.g. an ice chest full of frozen venizon ;)

After I put in the passenger name, I could see all the a la carte add-ons and fees. So FYI here's what I found on the Naples to Palermo route, 2 months from now:
$18.48 "Basic" fare incl 40x25x20 personal item (not weighed)
+ $27.50 “Regular” bundle with carry-on, reserved seat, priority boarding

+ $38.50 add “Plus” bundle with 20kg checked bag, reserved seat, and free airport check-in

+ $16.50 add 10 kg 55x40x20 carry-on OR checked bag
+ $27.50 add 20 kg 120x120x80 checked bag

+ $10-$23 add reserved seat, price depends on the row.