My friend and I decided not to go to the city center of Frankfurt tonight

We were supposed to go to the city center to have a nice dinner together as it has been quite a long time (1) since we saw each other. However, since public transportation will cost (2) over 9 euros each (extremely expensive) we decided to stay at home rather than go out. We will buy some nice ingredients at a shop, which is called REWE, located near my home (3). As we are going to travel a lot this weekend we thought it might be better to stay at home and save some money for our trips to Heidelberg and Marburg. Unfortunately there are not many things to do and see in the city where I live. So I will (4) help my friend to make his travel plans to Czech Republic and Hungary.

1. Or you could say "it has been quite some time" Also, you could say "since we saw each other'. You will hear more 'since we've seen each other"

2. What you wrote is fine, but my ear wants to hear ...'since public transportation "would" cost over 9 euros...'

3. I love REWE! You can use "which" in this sense, but I think you would hear people saying ...at a shop called REWE, located near my home".

4. Does your friend live in the same city as you? If so, then the last sentence would make more sense if you included the article in the previous sentence (...and see in the city where we live). Otherwise the last sentence sounds a bit akward. Or is the last sentence simply a new topic? In that case, the "so" would need a pause. Does that make sense?

nice work!