The cost of flying with surfboards varies by airline. Most airlines will take your surfboard for a fee, but there are some airlines that do not accept surfboards as checked baggage.

When checking the regulations, be sure to note how many surfboards are accepted in each boardbag, as some airlines have a limit as to the number of boards that can be taken. There are also limits on the size of boards, so some longboards and SUPs cannot be taken with some airlines.

When flying with your surfboard, always use a proper travel boardbag with more padding than a typical day bag. Pad the rails well, and it is also recommended to use tail and nose covers to protect the boards while handling. You’ll want to mark the bags as « Top Load Only » and « Fragile ». At most airports, your surfboard will arrive at the oversize luggage carousel, rather than the normal luggage carousel with suitcases. (Source; Airline-baggage-fees.com)

Ryanair
A surfboard may be carried in the hold of the aircraft in addition to your personal checked baggage allowance up to a limit of 20 kilos per item upon payment of a fee per item.
Ryanair Surfboard Fee 2021 – Large Sports Item
€55 / £55 (online)
€65 / £65 (at airport)
Fee charged per Item/ Per one way flight. A maximum weight of 20kg per item
Any sporting and musical item weighing over the 20 kilos allowance will be charged for the excess at the applicable excess baggage rate per kilo (currently set at 11 GBP / EUR per kilo).

EasyJet
An additional non-refundable fee is charged per item of surfboards (sports equipment) as follows. Surfboards qualify as « Large Sports Equipment » under EasyJet rules.
EasyJet accepts Surfboards Equipment as Large Sporting Equipment.
EASYJET LARGE SPORTS EQUIPMENT FEE – limit of 32kg weight allowance
Paid Online – £45 GBP
At Airport – £55 GBP
If purchasing standard baggage and sports equipment, the exact distribution of weight between baggage items doesn’t matter, subject to the maximum weights (32 kg per item) for health and safety reasons.

TUIfly
Surfboards are accepted on TUIfly flights for a fee of 65 EUR. The board bag is limited in weight up to a maximum of 30kg.
Be sure to register your surfboard through the TUIfly website or call center, in order to ensure enough space on the plane. If you do not register your surfboard ahead of time, it may be refused at check-in or be subject to additional fees.

Air France
Surfboards and longboards can be transported in the hold on Air France flights. Prior approval from Air France customer service department is required. You must submit your request at least 48 hours before your flight’s departure.
Depending on the size of the board and your travel dates, the following rules apply.
If your board is less than 300 cm / 118 inches in length and under 23 kg / 50 lb in weight, it is considered a standard checked baggage item and is included at no extra charge in your baggage allowance (except for Light and Basic fare tickets) if it is the only baggage that you want to check in (or one out of the 2 baggages allowed for the Business and La Première cabins.)

KLM
A surf board will be accepted on KLM flights as long as it is packed in a surf board bag. Be sure to note that it may get expensive if flying to/from Canada, the US, or Mexico, as a special surfboard fee will apply on those flights.
You can bring a surf board bag instead of a suitcase for flights operated by KLM, AIR FRANCE or Hop!
You need to reserve transportation of your surf equipment a minimum of 48 hours before departure by contacting KLM customer service.
Your surf equipment (including bag) is limited to max. 300 cm / 118 and may weigh max. 23 kg (50.5 lbs) in Economy Class, and max. 32 kg (70.5 lbs) in Business Class.

Air Canada
Surfboards are subject to a fee of $100.00 CA/US (plus applicable taxes) for carriage on Air Canada and Air Canada Rouge flights, as well as on Air Canada Express flights operated by Jazz, Sky Regional and Exploits Valley Air.

Source: Airline-baggage-fees.com