Learn Spanish (Spain): Este piso es igual de caro que tu casa. - This apartment is just as expensive as your house.

In Spanish, when you use the phrase igual de to express "just as", the second part of the comparison uses que. If you used tan to say "as expensive as", you would use como instead (tan caro como).

Why is it caro (masculine) when it is being compared to tu casa (feminine)?

The adjective caro describes the subject of the sentence, Este piso (masculine), not tu casa. If the sentence were flipped to start with the house, it would be Tu casa es igual de cara que este piso.

Is there a difference in meaning between igual de caro que and tan caro como?

They mean exactly the same thing: "just as expensive as". The structure igual de [adjective] que is just another very common way to make equal comparisons.

I have learned that piso means "floor". Is it the normal word for "apartment" here?

Yes, in Spain, piso is the everyday word for a residential "apartment" or "flat". It also means the "floor" or "level" of a building, so you just have to rely on context.