Can you pay a cost without getting the effect?

I'm pretty sure you can but I just need to make sure.

The situation is, I have demise on the field, no other cards on either players side of the field. Can I pay 2000 and blow up nothing? And assuming I can, can I do it multiple times.

You cannot activate an effect that will do "nothing". There must be at least one card other than Demise on the field, so obviously, you can only do it once.

No, like Level Modulation I believe it's called. You cant have both people draw a card unless you have a LV monster in your hand.

ok,

I'm still a little confused on this. I know if skill drain is active you can pay costs(for example you can discard for tribe, even though his effect will be negated). So whats the difference between the two?

keep in mind with tribe, you can only declare a type on the field, you can't just discard and declare something not on the field.

secondly, with tribe, you are activating the effect, but skill drain negates it. the situation youre thinking about is more like my first paragraph. activating an effect when there's nothing for it to target.

To answer your Tribe Infecting Virus parallel, that would only work if you had something to destroy with its effect. This is the exact same ruling with Demise. You cannot discard a card for Tribe Infecting Virus to destroy a type that is not on the field. (So if you want to empty your hand for some strange reason, keep declaring AQUA while Skill Drain is on the field. You'll get to discard, TIV just won't be destroyed.)

If Demise has nothing to send to the graveyard, you cannot activate its effect.

thanks everyone for clearing that up for me.

Another "angle" to this question is targeting eligibility versus how an effect resolves.

Example:
Player A has Silent Swordsman LV5 in ATK position. It is Player B's turn and he activates Enemy Controller and pays the cost of tributing a monster on his side of the field to take control of Silent Swordsman LV5. SSLV5 is a legal target of Enemy Controller. However when Enemy Controller resolves SSLV5 does not switch control because SSLV5 is unaffected by the effects of your opponents spell cards. So in this example you are paying the cost but the end result is not what you wanted to achieve.

Bad example. Anyone who has played this game for more than a few months knows that Silent Swordsman LV5 is unaffected by opponents Spell Card Effects, so the only thing that they could be hoping to achieve is the tributing off of a monster from the field, or removing a useless card from their Spell/Trap Zone.

So, I would tend to believe that a player "knows" exactly what they are doing when they play the cards you used in your scenario.

I had to answer different variations to this question a couple of different times during the SJC last weekend. I really don't think that there was a giant conspiracy to deliberately waste my time on all accounts. Even if it were true, motive in the situation is irrelevant as well as credit hours playing the game. Regardless of intent in the example the point remains that cost of a legal activation is nonrefundable.

Can you pay a cost without getting the effect? If you are trying to activate a card that can do nothing then the answer is no. In my example the answer is yes. Enemy Controller resolves with no change of control of the targeted monster.

You're using a "play on words". The monster is simply "Unaffected". It doesnt mean that under another set of circumstances another monster would not have switched control. It just means in this circumstance, it resolves without effect, but not because there was never another monster to target.