what should I pay?

These prices are based on mint on card (moc) or mint in box (mib) Tamagotchis. This means they have never been removed, or played with. You can also buy Tamagotchis in a used condition, this is a cheaper way of collecting them.
Regular Tams
The prices for a generation 1 tam will vary from $25 up till $45, depending on how rare the colour is.
The generation 2 Tamagotchi will cost you about the same as a generation 1.
Special Tams
Tamagotchi Angel
You can buy an Angelgotch for about $30, just keep an eye out on places like ebay, they aren't very rare. The tamagotchi Angel European and American versions do sell for higher prices, expect to pay about $55,- or more for those
Tamagotchi Wave Ufo
You won't see this Wave Ufo often on Ebay, but if you find one you should be able to buy it for $20,-.
Tamagotchi Osuchi
I'd recommend buying Osuchi and Mesuchi together for around $35,- to $50,- The more rare colours like clear green and orange usually go for higher prices.
Tamagotchi Morino
The Morino Tama costs about $40,-.
Tamagotchi Mothra
You can get your hands on a blue Mothra Tamagotchi for $35,- The green version is harder to get and will cost about $45,-
Tamagotchi Digimon
Digimons seem to be in high demand nowadays, you can purchase them for $50,- up to $150,-.
Tamagotchi Ocean
The Oceangotchi will cost you about $50,-, but you might be lucky and find one cheaper. The American version called Tamagotchi Ocean is very hard to find (especially MOC), I'm not sure for how much it goes, but expect to pay more than $100,-.
Rare Tams
Tamagotchi Genjintch
The Genjintch will sell for about $35,- It isn't super rare.
Tamagotchi Tamaotch You won't find a Tamaotch often, but if you do you can buy it for about $40,-

Arukotch used to cost about $45,- (until 2006)
But since they are becoming very rare their price can go up to even $300,- (O_o)

Tamagotchi Santaclautchi
Santaclautch is worth about $200,-, but you might be able to buy one for less.
Tamagotchi Devil
DevilGotchi will cost you a lot of money, the prices range from $200,- till $250,-
Tamagotchi Yasashii
The Yasashii is the most expensive Tama, it costs $200,- till $400,- for a mip.