* Italy now houses almost 100,000 migrants

* Total 2015 arrivals seen at 170,000, in line with 2014

* Spending on housing migrants seen at 1.16 bln euros this year

By Steve Scherer

ROME, Oct 21 (Reuters) - Italy sent 70 Syrian and Eritrean asylum seekers to Sweden and Finland on Wednesday as part of a European Union relocation plan meant to ease the burden of taking in 300,000 migrants in less than two years.

Fifty-one Eritreans and 19 Syrians who survived an often deadly journey across the desert and sea to reach Europe boarded a plane in Rome to move north two weeks after the first departures under the same EU plan.

"We have dozens of other migrants ready to leave in coming days," Interior Minister Angelino Alfano said at the airport.

Later in the day, the Interior Ministry published a study saying more than 99,000 migrants are now being housed in Italy at an estimated cost to the state of about 1.16 billion euros ($1.32 billion) in 2015, almost double last year's spending.

Relocation costs are paid from the EU budget, with 6,000 euros per head going to the country that accepts the refugees and 500 euros to the country transferring them.

Of over half a million refugees and migrants to arrive in Europe by boat across the Mediterranean this year, 136,000 have come through Italy, making it a frontline state in the continent's biggest refugee crisis since World War Two. Only Greece has seen more arrivals.

The EU aims to relocate almost 40,000 from Italy over two years, a target Alfano said was "absolutely doable". About 170,000 arrivals are foreseen for all of 2015, the same number as last year, according to the ministry study.

The crisis has caused bitter disputes between member states about how to share out responsibility.

Italy wants spending on migrants to be excluded from EU budget deficit calculations, Domenico Manzione, a junior minister at the Interior Ministry, told Reuters.

"It seems to be a legitimate request to which Europe, which seems unwilling, would do well to agree to," Manzione said.

Almost three-quarters of migrants here are living in temporary structures. Of the 61,545 asylum requests received through Oct. 9, 46,490 have been examined and just over half were rejected, the Interior Ministry study said.

EU countries have urged Italy to ramp up deportations of those who do not qualify for asylum. Over the past seven years, Italy has deported fewer than 28,000 using EU funds, it said.

While the study did not give an overall number of repatriations from Italy, a member of its working panel said the number of deportations funded by the EU was greater than those funded by Italy alone. ($1 = 0.8817 euros) (Additional reporting by Gabriele Pileri; Editing by Tom Heneghan)