First of, I'm glad it works out like in reality where GC needs to generate about £3M with player sales to even the books.The general answer for your question about attendance growth is, yes this is realistic. I will go a little deeper though. Until 2007 GC played in Hardturm Stadium and moved to Letzigrund Stadium afterwards. Hardturm was a stadium built only for football while Letzigrund is used as track and field and concert venue. It isn't built just for football therefore it's unattractive. And, from a GC point of view, it is the stadium of the local rival FCZ. A new stadium solely for football purposes was planned way back in 2003 but never realised. So GC has to play in the stadium of the enemy which is not mainly a football stadium.
Then there's Zurich itself. It isn't really a football city. It's not "cool" to go to football in Zurich. At least for the most part. And there is competition from Ice Hockey where the local Hockey club has an attendance average similar to FC Zurich but far better than GC. Then there's the problem of GC not being a Zurich club for the most part. GC is located in Niederhasli, 15 miles outside of Zurich. The general opinion in Zurich is, FC Zurich is the Zurich-City club and is also seen as the working class club while GC isn't really Zurich and is the club for the rich. You can see season ticket holders on this graph here: http://blog.tagesanzeiger.ch/datenblog/index.php/4463/die-fan-hochburgen-der-zuercher-clubs
There you can see that Zurich City is FCZ country, while in the suburbs it's more GC, especially at the so-called "gold coast" along the eastern shore of Lake Zurich and near Niederhasli, north of Zurich.
A lot of fans actually make fun of GC for having such low attendance figures. If you have figures above 10k consider yourself lucky. In fact here's a chart of the attendance figures of the last 40 years: http://www.gc-zone.ch/statistik_zuschauer.pdf (page 2). You see they managed to get above a 10k average only once in 40 years.
To your second question, for some clubs selling players worth of £3-4M per year is not uncommon. Basel sold players for £11M this summer plus Mo Salah to Chelsea for about £14M last winter. But that's Basel. They made a turnover of over £60M last year. A big part of that was player sales.
Young Boys are privately owned by the Rhis brothers, founders of Phonak (hearing aids, former cycling team). You probably haven't heard of them. So there is no need to sell big but they actually managed to sell players worth in between £3 to £8M in the last 5 seasons. (see here: http://www.transfermarkt.co.uk/bsc-young-boys/alletransfers/verein/452)
FCZ has a chairman who brings funds to the club. This is not reflected in the DB because FCZ generates enough money in FM15 to be realistic. I'm still thinking about adding their president as underwriter sugar daddy. But they sold players for a lot of money. Not every season but still: http://www.transfermarkt.co.uk/fc-zurich/alletransfers/verein/260
And as I said, GC needs money to balance the books. They have men who provide funds but not much. Look at the sales of the previous seasons: http://www.transfermarkt.co.uk/grasshopper-club-zurich/alletransfers/verein/504
GC sold players for at least £2.5M almost every year. Even going as high as £8M.
Other clubs usually don't have sales in that reagion although on occasion there is one player who is sold for millions, like Oscar Scarione from St. Gallen to Kasimpasa for about £3M in 2013. To conclude, yes those values are realistic. Please keep in mind it's very hard and frustrating to get accurate financial data in Switzerland. Most clubs keep it secret. Only Aarau, Basel and Thun make their annual business reports public. The figures from other clubs are taken from newspaper articles or are guessed. In the case of GC there were multiple articles on their financials because it's a theme every year.
I'm asking out of interest, which leagues do you compare and on what level? Because the level on the pitch and the financial level in Switzerland are two different topics.
Thanks btw for playing in the Swiss league and making a thread about it on here. If you have any feedback or find mistakes, don't hesitate to contact me.