Can I see all of Southeast Asia in 2 weeks?

Technically, you could fly between capitals, but you would see very little. Stick to two countries maximum for a 2-week trip to ensure you actually enjoy the locations.

Is it cheaper to stay longer?

Yes. Your biggest costs are international flights and transit. By moving slower, you save on transport and can negotiate better weekly rates for accommodation.

How long to spend in Southeast Asia

Spend at least 3 weeks to see the highlights of 2-3 countries without constant travel fatigue. If you want to cover the major sites of the entire region, allocate 8 to 10 weeks to maintain a sustainable pace.

Define your geography. Decide if you are focusing on the 'Banana Pancake Trail' (Thailand, Laos, Vietnam, Cambodia) or incorporating the islands (Indonesia, Philippines, Malaysia). Trying to do both in under a month will result in spending more time at airports than on the ground.
Account for transit time. Budget one full day for every time you cross an international border or take a domestic flight. Between flight delays, taxi transfers to city centers, and hotel check-ins, you lose more hours than you think.
Apply the '3-Night Rule'. Don't move hotels every night. Stay in one place for a minimum of 3 nights to actually experience the culture and recover from the heat, rather than just packing and unpacking your bag.
Factor in the 'Burnout Buffer'. Add 3-5 'do nothing' days to your trip. Southeast Asian humidity and constant sensory input are exhausting; you will need downtime to sit by a pool or sleep in without feeling guilty.

Can I see all of Southeast Asia in 2 weeks?: Technically, you could fly between capitals, but you would see very little. Stick to two countries maximum for a 2-week trip to ensure you actually enjoy the locations.
Is it cheaper to stay longer?: Yes. Your biggest costs are international flights and transit. By moving slower, you save on transport and can negotiate better weekly rates for accommodation.